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\pdf_author "Marek Nečada"
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\begin_body

\begin_layout Section
Finite systems
\end_layout

\begin_layout Subsection
Motivation/intro
\end_layout

\begin_layout Standard
The basic idea of MSTMM is quite simple: the driving electromagnetic field
 incident onto a scatterer is expanded into a vector spherical wavefunction
 (VSWF) basis in which the single scattering problem is solved, and the
 scattered field is then re-expanded into VSWFs centered at the other scatterers.
 Repeating the same procedure with all (pairs of) scatterers yields a set
 of linear equations, solution of which gives the coefficients of the scattered
 field in the VSWF bases.
 Once these coefficients have been found, one can evaluate various quantities
 related to the scattering (such as cross sections or the scattered fields)
 quite easily.
 
\end_layout

\begin_layout Standard
The expressions appearing in the re-expansions are fairly complicated, and
 the implementation of MSTMM is extremely error-prone also due to the various
 conventions used in the literature.
 Therefore although we do not re-derive from scratch the expressions that
 can be found elsewhere in literature, we always state them explicitly in
 our convention.
\end_layout

\begin_layout Subsection
Single-particle scattering
\end_layout

\begin_layout Standard
In order to define the basic concepts, let us first consider the case of
 EM radiation scattered by a single particle.
 We assume that the scatterer lies inside a closed sphere 
\begin_inset Formula $\particle$
\end_inset

, the space outside this volume 
\begin_inset Formula $\medium$
\end_inset

 is filled with an homogeneous isotropic medium with relative electric permittiv
ity 
\begin_inset Formula $\epsilon(\vect r,\omega)=\epsbg(\omega)$
\end_inset

 and magnetic permeability 
\begin_inset Formula $\mu(\vect r,\omega)=\mubg(\omega)$
\end_inset

, and that the whole system is linear, i.e.
 the material properties of neither the medium nor the scatterer depend
 on field intensities.
 Under these assumptions, the EM fields 
\begin_inset Formula $\vect{\Psi}=\vect E,\vect H$
\end_inset

 in 
\begin_inset Formula $\medium$
\end_inset

 must satisfy the homogeneous vector Helmholtz equation together with the
 transversality condition 
\begin_inset Formula 
\begin{equation}
\left(\nabla^{2}+k^{2}\right)\Psi\left(\vect r,\vect{\omega}\right)=0,\quad\nabla\cdot\vect{\Psi}\left(\vect r,\vect{\omega}\right)=0\label{eq:Helmholtz eq}
\end{equation}

\end_inset

 
\begin_inset Note Note
status open

\begin_layout Plain Layout
frequency-space Maxwell's equations
\begin_inset Formula 
\begin{align*}
\nabla\times\vect E\left(\vect r,\omega\right)-ik\eta_{0}\eta\vect H\left(\vect r,\omega\right) & =0,\\
\eta_{0}\eta\nabla\times\vect H\left(\vect r,\omega\right)+ik\vect E\left(\vect r,\omega\right) & =0.
\end{align*}

\end_inset


\end_layout

\end_inset

 
\begin_inset Note Note
status open

\begin_layout Plain Layout
todo define 
\begin_inset Formula $\Psi$
\end_inset

, mention transversality
\end_layout

\end_inset

 with 
\begin_inset Formula $k=k\left(\omega\right)=\omega\sqrt{\mubg(\omega)\epsbg(\omega)}/c_{0}$
\end_inset

, as can be derived from the Maxwell's equations 
\begin_inset CommandInset citation
LatexCommand cite
after "???"
key "jackson_classical_1998"
literal "false"

\end_inset

.
 
\end_layout

\begin_layout Subsubsection
Spherical waves
\end_layout

\begin_layout Standard
Equation 
\begin_inset CommandInset ref
LatexCommand ref
reference "eq:Helmholtz eq"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can be solved by separation of variables in spherical coordinates to give
 the solutions – the 
\emph on
regular
\emph default
 and 
\emph on
outgoing
\emph default
 vector spherical wavefunctions (VSWFs) 
\begin_inset Formula $\vswfrtlm{\tau}lm\left(k\vect r\right)$
\end_inset

 and 
\begin_inset Formula $\vswfouttlm{\tau}lm\left(k\vect r\right)$
\end_inset

, respectively, defined as follows:
\begin_inset Formula 
\begin{align}
\vswfrtlm 1lm\left(k\vect r\right) & =j_{l}\left(kr\right)\vsh 1lm\left(\uvec r\right),\nonumber \\
\vswfrtlm 2lm\left(k\vect r\right) & =\frac{1}{kr}\frac{\ud\left(krj_{l}\left(kr\right)\right)}{\ud\left(kr\right)}\vsh 2lm\left(\uvec r\right)+\sqrt{l\left(l+1\right)}\frac{j_{l}\left(kr\right)}{kr}\vsh 3lm\left(\uvec r\right),\label{eq:VSWF regular}
\end{align}

\end_inset


\begin_inset Formula 
\begin{align}
\vswfouttlm 1lm\left(k\vect r\right) & =h_{l}^{\left(1\right)}\left(kr\right)\vsh 1lm\left(\uvec r\right),\nonumber \\
\vswfouttlm 2lm\left(k\vect r\right) & =\frac{1}{kr}\frac{\ud\left(krh_{l}^{\left(1\right)}\left(kr\right)\right)}{\ud\left(kr\right)}\vsh 2lm\left(\uvec r\right)+\sqrt{l\left(l+1\right)}\frac{h_{l}^{\left(1\right)}\left(kr\right)}{kr}\vsh 3lm\left(\uvec r\right),\label{eq:VSWF outgoing}\\
 & \tau=1,2;\quad l=1,2,3,\dots;\quad m=-l,-l+1,\dots,+l,\nonumber 
\end{align}

\end_inset

where 
\begin_inset Formula $\vect r=r\uvec r$
\end_inset

, 
\begin_inset Formula $j_{l}\left(x\right),h_{l}^{\left(1\right)}\left(x\right)=j_{l}\left(x\right)+iy_{l}\left(x\right)$
\end_inset

 are the regular spherical Bessel function and spherical Hankel function
 of the first kind, respectively, as in [DLMF §10.47], and 
\begin_inset Formula $\vsh{\tau}lm$
\end_inset

 are the 
\emph on
vector spherical harmonics
\emph default

\begin_inset Formula 
\begin{align}
\vsh 1lm\left(\uvec r\right) & =\frac{1}{\sqrt{l\left(l+1\right)}}\nabla\times\left(\vect r\ush lm\left(\uvec r\right)\right)=\frac{1}{\sqrt{l\left(l+1\right)}}\nabla\ush lm\left(\uvec r\right)\times\vect r,\nonumber \\
\vsh 2lm\left(\uvec r\right) & =\frac{1}{\sqrt{l\left(l+1\right)}}r\nabla\ush lm\left(\uvec r\right),\nonumber \\
\vsh 3lm\left(\uvec r\right) & =\uvec r\ush lm\left(\uvec r\right).\label{eq:vector spherical harmonics definition}
\end{align}

\end_inset

In our convention, the (scalar) spherical harmonics 
\begin_inset Formula $\ush lm$
\end_inset

 are identical to those in [DLMF 14.30.1], i.e.
\begin_inset Formula 
\[
\ush lm=\left(\frac{\left(l-m\right)!\left(2l+1\right)}{4\pi\left(l+m\right)!}\right)^{\frac{1}{2}}e^{im\phi}\dlmfFer lm\left(\cos\theta\right)
\]

\end_inset

where importantly, the Ferrers functions 
\begin_inset Formula $\dlmfFer lm$
\end_inset

 defined as in [DLMF §14.3(i)] do already contain the Condon-Shortley phase
 
\begin_inset Formula $\left(-1\right)^{m}$
\end_inset

.
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO názornější definice.
\end_layout

\end_inset

 
\end_layout

\begin_layout Standard
The convention for VSWFs used here is the same as in 
\begin_inset CommandInset citation
LatexCommand cite
key "kristensson_spherical_2014"
literal "false"

\end_inset

; over other conventions used elsewhere in literature, it has several fundamenta
l advantages – most importantly, the translation operators introduced later
 in eq.
 
\begin_inset CommandInset ref
LatexCommand ref
reference "eq:translation op def"
plural "false"
caps "false"
noprefix "false"

\end_inset

 are unitary, and it gives the simplest possible expressions for power transport
 and cross sections without additional 
\begin_inset Formula $l,m$
\end_inset

-dependent factors (for that reason, we also call our VSWFs as 
\emph on
power-normalised
\emph default
).
 Power-normalisation and unitary translation operators are possible to achieve
 also with real spherical harmonics – such a convention is used in 
\begin_inset CommandInset citation
LatexCommand cite
key "kristensson_scattering_2016"
literal "false"

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
Its solutions (TODO under which conditions? What vector space do the SVWFs
 actually span? Check Comment 9.2 and Appendix f.9.1 in Kristensson)
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO small note about cartesian multipoles, anapoles etc.
 (There should be some comparing paper that the Russians at META 2018 mentioned.)
\end_layout

\end_inset


\end_layout

\begin_layout Subsubsection
T-matrix definition
\end_layout

\begin_layout Standard
The regular VSWFs 
\begin_inset Formula $\vswfrtlm{\tau}lm\left(k\vect r\right)$
\end_inset

 constitute a basis for solutions of the Helmholtz equation 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Helmholtz eq"
plural "false"
caps "false"
noprefix "false"

\end_inset

 inside a ball 
\begin_inset Formula $\openball 0{R^{>}}$
\end_inset

 with radius 
\begin_inset Formula $R^{>}$
\end_inset

 and center in the origin; however, if the equation is not guaranteed to
 hold inside a smaller ball 
\begin_inset Formula $B_{0}\left(R\right)$
\end_inset

 around the origin (typically due to presence of a scatterer), one has to
 add the outgoing VSWFs 
\begin_inset Formula $\vswfrtlm{\tau}lm\left(k\vect r\right)$
\end_inset

 to have a complete basis of the solutions in the volume 
\begin_inset Formula $\openball 0{R^{>}}\backslash B_{0}\left(R\right)$
\end_inset

.
\begin_inset Note Note
status open

\begin_layout Plain Layout
Vnitřní koule uzavřená? Jak se řekne mezikulí anglicky?
\end_layout

\end_inset


\end_layout

\begin_layout Standard
The single-particle scattering problem at frequency 
\begin_inset Formula $\omega$
\end_inset

 can be posed as follows: Let a scatterer be enclosed inside the ball 
\begin_inset Formula $B_{0}\left(R\right)$
\end_inset

 and let the whole volume 
\begin_inset Formula $\openball 0{R^{>}}\backslash B_{0}\left(R\right)$
\end_inset

 be filled with a homogeneous isotropic medium with wave number 
\begin_inset Formula $k\left(\omega\right)$
\end_inset

.
 Inside this volume, the electric field can be expanded as
\begin_inset Note Note
status open

\begin_layout Plain Layout
doplnit frekvence a polohy
\end_layout

\end_inset


\begin_inset Formula 
\begin{equation}
\vect E\left(\omega,\vect r\right)=\sum_{\tau=1,2}\sum_{l=1}^{\infty}\sum_{m=-l}^{+l}\left(\rcoefftlm{\tau}lm\vswfrtlm{\tau}lm\left(k\vect r\right)+\outcoefftlm{\tau}lm\vswfouttlm{\tau}lm\left(k\vect r\right)\right).\label{eq:E field expansion}
\end{equation}

\end_inset

If there was no scatterer and 
\begin_inset Formula $B_{0}\left(R_{<}\right)$
\end_inset

 was filled with the same homogeneous medium, the part with the outgoing
 VSWFs would vanish and only the part 
\begin_inset Formula $\vect E_{\mathrm{inc}}=\sum_{\tau lm}\rcoefftlm{\tau}lm\vswfrtlm{\tau}lm$
\end_inset

 due to sources outside 
\begin_inset Formula $\openball 0R$
\end_inset

 would remain.
 Let us assume that the 
\begin_inset Quotes eld
\end_inset

driving field
\begin_inset Quotes erd
\end_inset

 is given, so that presence of the scatterer does not affect 
\begin_inset Formula $\vect E_{\mathrm{inc}}$
\end_inset

 and is fully manifested in the latter part, 
\begin_inset Formula $\vect E_{\mathrm{scat}}=\sum_{\tau lm}\outcoefftlm{\tau}lm\vswfouttlm{\tau}lm$
\end_inset

.
 We also assume that the scatterer is made of optically linear materials,
 and hence reacts on the incident field in a linear manner.
 This gives a linearity constraint between the expansion coefficients
\begin_inset Formula 
\begin{equation}
\outcoefftlm{\tau}lm=\sum_{\tau'l'm'}T_{\tau lm}^{\tau'l'm'}\rcoefftlm{\tau'}{l'}{m'}\label{eq:T-matrix definition}
\end{equation}

\end_inset

where the 
\begin_inset Formula $T_{\tau lm}^{\tau'l'm'}=T_{\tau lm}^{\tau'l'm'}\left(\omega\right)$
\end_inset

 are the elements of the 
\emph on
transition matrix,
\emph default
 a.k.a.
 
\begin_inset Formula $T$
\end_inset

-matrix.
 It completely describes the scattering properties of a linear scatterer,
 so with the knowledge of the 
\begin_inset Formula $T$
\end_inset

-matrix, we can solve the single-patricle scatering prroblem simply by substitut
ing appropriate expansion coefficients 
\begin_inset Formula $\rcoefftlm{\tau'}{l'}{m'}$
\end_inset

 of the driving field into 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:T-matrix definition"
plural "false"
caps "false"
noprefix "false"

\end_inset

.
 The outgoing VSWF expansion coefficients 
\begin_inset Formula $\outcoefftlm{\tau}lm$
\end_inset

 are the effective induced electric (
\begin_inset Formula $\tau=2$
\end_inset

) and magnetic (
\begin_inset Formula $\tau=1$
\end_inset

) multipole polarisation amplitudes of the scatterer, and this is why we
 sometimes refer to the corresponding VSWFs as the electric and magnetic
 VSWFs.
 
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO mention the pseudovector character of magnetic VSWFs.
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
TOOD H-field expansion here?
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Formula $T$
\end_inset

-matrices of particles with certain simple geometries (most famously spherical)
 can be obtained analytically 
\begin_inset CommandInset citation
LatexCommand cite
key "kristensson_scattering_2016,mie_beitrage_1908"
literal "false"

\end_inset

, but in general one can find them numerically by simulating scattering
 of a regular spherical wave 
\begin_inset Formula $\vswfouttlm{\tau}lm$
\end_inset

 and projecting the scattered fields (or induced currents, depending on
 the method) onto the outgoing VSWFs 
\begin_inset Formula $\vswfrtlm{\tau}{'l'}{m'}$
\end_inset

.
 In practice, one can compute only a finite number of elements with a cut-off
 value 
\begin_inset Formula $L$
\end_inset

 on the multipole degree, 
\begin_inset Formula $l,l'\le L$
\end_inset

, see below.
 We typically use the scuff-tmatrix tool from the free software SCUFF-EM
 suite 
\begin_inset CommandInset citation
LatexCommand cite
key "reid_efficient_2015,SCUFF2"
literal "false"

\end_inset

.
 Note that older versions of SCUFF-EM contained a bug that rendered almost
 all 
\begin_inset Formula $T$
\end_inset

-matrix results wrong; we found and fixed the bug and from upstream version
 xxx onwards, it should behave correctly.
 
\end_layout

\begin_layout Subsubsection
T-matrix compactness, cutoff validity
\end_layout

\begin_layout Standard
The magnitude of the 
\begin_inset Formula $T$
\end_inset

-matrix elements depends heavily on the scatterer's size compared to the
 wavelength.
 Fortunately, the 
\begin_inset Formula $T$
\end_inset

-matrix of a bounded scatterer is a compact operator [REF???], so from certain
 multipole degree onwards, 
\begin_inset Formula $l,l'>L$
\end_inset

, the elements of the 
\begin_inset Formula $T$
\end_inset

-matrix are negligible, so truncating the 
\begin_inset Formula $T$
\end_inset

-matrix at finite multipole degree 
\begin_inset Formula $L$
\end_inset

 gives a good approximation of the actual infinite-dimensional itself.
 If the incident field is well-behaved, i.e.
 the expansion coefficients 
\begin_inset Formula $\rcoefftlm{\tau'}{l'}{m'}$
\end_inset

 do not take excessive values for 
\begin_inset Formula $l'>L$
\end_inset

, the scattered field expansion coefficients 
\begin_inset Formula $\outcoefftlm{\tau}lm$
\end_inset

 with 
\begin_inset Formula $l>L$
\end_inset

 will also be negligible.
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO when it will not be negligible
\end_layout

\end_inset


\end_layout

\begin_layout Standard
A rule of thumb to choose the 
\begin_inset Formula $L$
\end_inset

 with desired 
\begin_inset Formula $T$
\end_inset

-matrix element accuracy 
\begin_inset Formula $\delta$
\end_inset

 can be obtained from the spherical Bessel function expansion around zero,
 TODO.
 
\end_layout

\begin_layout Subsubsection
Power transport
\end_layout

\begin_layout Standard
For convenience, let us introduce a short-hand matrix notation for the expansion
 coefficients and related quantities, so that we do not need to write the
 indices explicitly; so for example, eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:T-matrix definition"
plural "false"
caps "false"
noprefix "false"

\end_inset

 would be written as 
\begin_inset Formula $\outcoeffp{}=T\rcoeffp{}$
\end_inset

, where 
\begin_inset Formula $\rcoeffp{},\outcoeffp{}$
\end_inset

 are column vectors with the expansion coefficients.
 Transposed and complex-conjugated matrices are labeled with the 
\begin_inset Formula $\dagger$
\end_inset

 superscript.
\end_layout

\begin_layout Standard
With this notation, we state an important result about power transport,
 derivation of which can be found in 
\begin_inset CommandInset citation
LatexCommand cite
after "sect. 7.3"
key "kristensson_scattering_2016"
literal "true"

\end_inset

.
 Let the field in 
\begin_inset Formula $\openball 0{R^{>}}\backslash B_{0}\left(R\right)$
\end_inset

 have expansion as in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:E field expansion"
plural "false"
caps "false"
noprefix "false"

\end_inset

.
 Then the net power transported from 
\begin_inset Formula $B_{0}\left(R\right)$
\end_inset

 to 
\begin_inset Formula $\openball 0{R^{>}}\backslash B_{0}\left(R\right)$
\end_inset

 via by electromagnetic radiation is
\begin_inset Formula 
\begin{equation}
P=\frac{1}{2k^{2}\eta_{0}\eta}\left(\Re\left(\rcoeffp{}^{\dagger}\outcoeffp{}\right)+\left\Vert \outcoeffp{}\right\Vert ^{2}\right)=\frac{1}{2k^{2}\eta_{0}\eta}\rcoeffp{}^{\dagger}\left(\Tp{}^{\dagger}\Tp{}+\frac{\Tp{}^{\dagger}+\Tp{}}{2}\right)\rcoeffp{}.\label{eq:Power transport}
\end{equation}

\end_inset

In realistic scattering setups, power is transferred by radiation into 
\begin_inset Formula $B_{0}\left(R\right)$
\end_inset

 and absorbed by the enclosed scatterer, so 
\begin_inset Formula $P$
\end_inset

 is negative and its magnitude equals to power absorbed by the scatterer.
\end_layout

\begin_layout Subsubsection
Plane wave expansion
\end_layout

\begin_layout Standard
In many scattering problems considered in practice, the driving field is
 a plane wave.
 A transversal (
\begin_inset Formula $\vect k\cdot\vect E_{0}=0$
\end_inset

) plane wave propagating in direction 
\begin_inset Formula $\uvec k$
\end_inset

 with (complex) amplitude 
\begin_inset Formula $\vect E_{0}$
\end_inset

 can be expanded into regular VSWFs 
\begin_inset CommandInset citation
LatexCommand cite
after "7.???"
key "kristensson_scattering_2016"
literal "false"

\end_inset

 as
\begin_inset Formula 
\[
\vect E_{\mathrm{PW}}\left(\vect r,\omega\right)=\vect E_{0}e^{ik\uvec k\cdot\vect r}=\sum_{\tau,l,m}\rcoeffptlm{}{\tau}lm\left(\vect k,\vect E_{0}\right)\vswfrtlm{\tau}lm\left(k\vect r\right),
\]

\end_inset

with expansion coefficients
\begin_inset Formula 
\begin{eqnarray}
\rcoeffptlm{}1lm\left(\vect k,\vect E_{0}\right) & = & 4\pi i^{l}\vshD 1lm\left(\uvec k\right),\nonumber \\
\rcoeffptlm{}2lm\left(\vect k,\vect E_{0}\right) & = & -4\pi i^{l+1}\vshD 2lm\left(\uvec k\right).\label{eq:plane wave expansion}
\end{eqnarray}

\end_inset

where 
\begin_inset Formula $\vshD{\tau}lm$
\end_inset

 are the 
\begin_inset Quotes eld
\end_inset

dual
\begin_inset Quotes erd
\end_inset

 vector spherical harmonics defined by duality relation with the 
\begin_inset Quotes eld
\end_inset

usual
\begin_inset Quotes erd
\end_inset

 vector spherical harmonics 
\begin_inset Formula 
\begin{equation}
\iint\vshD{\tau'}{l'}{m'}\left(\uvec r\right)\cdot\vsh{\tau}lm\left(\uvec r\right)\,\ud\Omega=\delta_{\tau'\tau}\delta_{l'l}\delta_{m'm}\label{eq:dual vsh}
\end{equation}

\end_inset

(complex conjugation not implied in the dot product here).
 In our convention, we have
\begin_inset Formula 
\[
\vshD{\tau}lm\left(\uvec r\right)=\left(\vsh{\tau}lm\left(\uvec r\right)\right)^{*}=\left(-1\right)^{m}\vsh{\tau}{l-}m\left(\uvec r\right).
\]

\end_inset


\end_layout

\begin_layout Subsubsection
Cross-sections (single-particle)
\end_layout

\begin_layout Standard
With the 
\begin_inset Formula $T$
\end_inset

-matrix and expansion coefficients of plane waves in hand, we can state
 the expressions for cross-sections of a single scatterer.
 Assuming a non-lossy background medium, extinction, scattering and absorption
 cross sections of a single scatterer irradiated by a plane wave propagating
 in direction 
\begin_inset Formula $\uvec k$
\end_inset

 and (complex) amplitude 
\begin_inset Formula $\vect E_{0}$
\end_inset

 are 
\begin_inset CommandInset citation
LatexCommand cite
after "sect. 7.8.2"
key "kristensson_scattering_2016"
literal "true"

\end_inset


\begin_inset Formula 
\begin{eqnarray}
\sigma_{\mathrm{ext}}\left(\uvec k\right) & = & -\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\Re\left(\rcoeffp{}^{\dagger}\outcoeffp{}\right)=-\frac{1}{2k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\rcoeffp{}^{\dagger}\left(\Tp{}+\Tp{}^{\dagger}\right)\rcoeffp{},\label{eq:extincion CS single}\\
\sigma_{\mathrm{scat}}\left(\uvec k\right) & = & \frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\left\Vert \outcoeffp{}\right\Vert ^{2}=\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\rcoeffp{}^{\dagger}\left(\Tp{}^{\dagger}\Tp{}\right)\rcoeffp{},\label{eq:scattering CS single}\\
\sigma_{\mathrm{abs}}\left(\uvec k\right) & = & \sigma_{\mathrm{ext}}\left(\uvec k\right)-\sigma_{\mathrm{scat}}\left(\uvec k\right)=-\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\left(\Re\left(\rcoeffp{}^{\dagger}\outcoeffp{}\right)+\left\Vert \outcoeffp{}\right\Vert ^{2}\right)\nonumber \\
 &  & =\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\rcoeffp{}^{\dagger}\left(\Tp{}^{\dagger}\Tp{}+\frac{\Tp{}^{\dagger}+\Tp{}}{2}\right)\rcoeffp{},\label{eq:absorption CS single}
\end{eqnarray}

\end_inset

where 
\begin_inset Formula $\rcoeffp{}=\rcoeffp{}\left(\vect k,\vect E_{0}\right)$
\end_inset

 is the vector of plane wave expansion coefficients as in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:plane wave expansion"

\end_inset

.
 
\end_layout

\begin_layout Subsection
Multiple scattering
\begin_inset CommandInset label
LatexCommand label
name "subsec:Multiple-scattering"

\end_inset


\end_layout

\begin_layout Standard
If the system consists of multiple scatterers, the EM fields around each
 one can be expanded in analogous way.
 Let 
\begin_inset Formula $\mathcal{P}$
\end_inset

 be an index set labeling the scatterers.
 We enclose each scatterer in a ball 
\begin_inset Formula $B_{\vect r_{p}}\left(R_{p}\right)$
\end_inset

 such that the balls do not touch, 
\begin_inset Formula $B_{\vect r_{p}}\left(R_{p}\right)\cap B_{\vect r_{q}}\left(R_{q}\right)=\emptyset;p,q\in\mathcal{P}$
\end_inset

, 
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO bacha, musejí být uzavřené!
\end_layout

\end_inset

so there is a non-empty volume
\begin_inset Note Note
status open

\begin_layout Plain Layout
jaksetometuje?
\end_layout

\end_inset

 
\begin_inset Formula $\openball{\vect r_{p}}{R_{p}^{>}}\backslash B_{\vect r_{p}}\left(R_{p}\right)$
\end_inset

 around each one that contains only the background medium without any scatterers.
 Then the EM field inside each such volume can be expanded in a way similar
 to 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:E field expansion"
plural "false"
caps "false"
noprefix "false"

\end_inset

, using VSWFs with origins shifted to the centre of the volume:
\begin_inset Formula 
\begin{align}
\vect E\left(\omega,\vect r\right) & =\sum_{\tau=1,2}\sum_{l=1}^{\infty}\sum_{m=-l}^{+l}\left(\rcoeffptlm p{\tau}lm\vswfrtlm{\tau}lm\left(k\left(\vect r-\vect r_{p}\right)\right)+\outcoeffptlm p{\tau}lm\vswfouttlm{\tau}lm\left(k\left(\vect r-\vect r_{p}\right)\right)\right),\label{eq:E field expansion multiparticle}\\
 & \vect r\in\openball{\vect r_{p}}{R_{p}^{>}}\backslash B_{\vect r_{p}}\left(R_{p}\right).\nonumber 
\end{align}

\end_inset

Unlike the single scatterer case, the incident field coefficients 
\begin_inset Formula $\rcoeffptlm p{\tau}lm$
\end_inset

 here are not only due to some external driving field that the particle
 does not influence but they also contain the contributions of fields scattered
 from 
\emph on
all other scatterers
\emph default
: 
\begin_inset Formula 
\begin{equation}
\rcoeffp p=\rcoeffincp p+\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\tropsp pq\outcoeffp q\label{eq:particle total incident field coefficient a}
\end{equation}

\end_inset

where 
\begin_inset Formula $\rcoeffincp p$
\end_inset

 represents the part due to the external driving that the scatterers can
 not influence, and 
\begin_inset Formula $\tropsp pq$
\end_inset

 is a 
\emph on
translation operator
\emph default
 defined below in Sec.
 
\begin_inset CommandInset ref
LatexCommand ref
reference "subsec:Translation-operator"
plural "false"
caps "false"
noprefix "false"

\end_inset

, that contains the re-expansion coefficients of the outgoing waves in origin
 
\begin_inset Formula $\vect r_{q}$
\end_inset

 into regular waves in origin 
\begin_inset Formula $\vect r_{p}$
\end_inset

.
 For each scatterer, we also have its 
\begin_inset Formula $T$
\end_inset

-matrix relation as in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:T-matrix definition"
plural "false"
caps "false"
noprefix "false"

\end_inset

,
\begin_inset Formula 
\[
\outcoeffp q=T_{q}\rcoeffp q.
\]

\end_inset

Together with 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:particle total incident field coefficient a"
plural "false"
caps "false"
noprefix "false"

\end_inset

, this gives rise to a set of linear equations
\begin_inset Formula 
\begin{equation}
\outcoeffp p-T_{p}\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\tropsp pq\outcoeffp q=T_{p}\rcoeffincp p,\quad p\in\mathcal{P}\label{eq:Multiple-scattering problem}
\end{equation}

\end_inset

which defines the multiple-scattering problem.
 If all the 
\begin_inset Formula $p,q$
\end_inset

-indexed vectors and matrices (note that without truncation, they are infinite-d
imensional) are arranged into blocks of even larger vectors and matrices,
 this can be written in a short-hand form
\begin_inset Formula 
\begin{equation}
\left(I-T\trops\right)\outcoeff=T\rcoeffinc\label{eq:Multiple-scattering problem block form}
\end{equation}

\end_inset

where 
\begin_inset Formula $I$
\end_inset

 is the identity matrix, 
\begin_inset Formula $T$
\end_inset

is a block-diagonal matrix containing all the individual 
\begin_inset Formula $T$
\end_inset

-matrices, and 
\begin_inset Formula $\trops$
\end_inset

 contains the individual 
\begin_inset Formula $\tropsp pq$
\end_inset

matrices as the off-diagonal blocks, whereas the diagonal blocks are set
 to zeros.
\end_layout

\begin_layout Standard
In practice, the multiple-scattering problem is solved in its truncated
 form, in which all the 
\begin_inset Formula $l$
\end_inset

-indices related to a given scatterer 
\begin_inset Formula $p$
\end_inset

 are truncated as 
\begin_inset Formula $l\le L_{p}$
\end_inset

, laeving only 
\begin_inset Formula $N_{p}=2L_{p}\left(L_{p}+2\right)$
\end_inset

 different 
\begin_inset Formula $\tau lm$
\end_inset

-multiindices left.
 The truncation degree can vary for different scatterers (e.g.
 due to different physical sizes), so the truncated block 
\begin_inset Formula $\tropsp pq$
\end_inset

 has shape 
\begin_inset Formula $N_{p}\times N_{q}$
\end_inset

, not necessarily square.
 
\begin_inset Note Note
status open

\begin_layout Plain Layout
Such truncation of the translation operator 
\begin_inset Formula $\tropsp pq$
\end_inset

 is justified by the fact on the left, TODO
\end_layout

\end_inset


\end_layout

\begin_layout Standard
If no other type of truncation is done, there remain 
\begin_inset Formula $2L_{p}\left(L_{p}+2\right)$
\end_inset

 different 
\begin_inset Formula $\tau lm$
\end_inset

-multiindices for 
\begin_inset Formula $p$
\end_inset

-th scatterer, so that the truncated version of the matrix 
\begin_inset Formula $\left(I-T\trops\right)$
\end_inset

 is a square matrix with 
\begin_inset Formula $\left(\sum_{p\in\mathcal{P}}N_{p}\right)^{2}$
\end_inset

 elements in total.
 The truncated problem 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Multiple-scattering problem block form"
plural "false"
caps "false"
noprefix "false"

\end_inset

 can then be solved using standard numerical linear algebra methods (typically,
 by LU factorisation of the 
\begin_inset Formula $\left(I-T\trops\right)$
\end_inset

 matrix at a given frequency, and then solving with Gauss elimination for
 as many different incident 
\begin_inset Formula $\rcoeffinc$
\end_inset

 vectors as needed).
\end_layout

\begin_layout Standard
Alternatively, the multiple scattering problem can be formulated in terms
 of the regular field expansion coefficients,
\begin_inset Formula 
\begin{align*}
\rcoeffp p-\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\tropsp pqT_{q}\rcoeffp q & =\rcoeffincp p,\quad p\in\mathcal{P},\\
\left(I-\trops T\right)\rcoeff & =\rcoeffinc,
\end{align*}

\end_inset

but this form is less suitable for numerical calculations due to the fact
 that the regular VSWF expansion coefficients on both sides of the equation
 are typically non-negligible even for large multipole degree 
\begin_inset Formula $l$
\end_inset

, hence the truncation is not justified in this case.
 
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO less bulshit.
\end_layout

\end_inset


\end_layout

\begin_layout Subsubsection
Translation operator
\begin_inset CommandInset label
LatexCommand label
name "subsec:Translation-operator"

\end_inset


\end_layout

\begin_layout Standard
Let 
\begin_inset Formula $\vect r_{1},\vect r_{2}$
\end_inset

 be two different origins; a regular VSWF with origin 
\begin_inset Formula $\vect r_{1}$
\end_inset

 can be always expanded in terms of regular VSWFs with origin 
\begin_inset Formula $\vect r_{2}$
\end_inset

 as follows:
\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
\vswfrtlm{\tau}lm\left(k\left(\vect r-\vect r_{1}\right)\right)=\sum_{\tau'l'm'}\tropr_{\tau lm;\tau'l'm'}\left(k\left(\vect r_{2}-\vect r_{1}\right)\right)\vswfrtlm{\tau'}{l'}{m'}\left(\vect r-\vect r_{2}\right),\label{eq:regular vswf translation}
\end{equation}

\end_inset

where an explicit formula for the (regular) 
\emph on
translation operator
\emph default
 
\begin_inset Formula $\tropr$
\end_inset

 reads in eq.
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:translation operator"

\end_inset

 below.
 For singular (outgoing) waves, the form of the expansion differs inside
 and outside the ball 
\begin_inset Formula $\openball{\left\Vert \vect r_{2}-\vect r_{1}\right\Vert }{\vect r_{1}}:$
\end_inset

 
\begin_inset Formula 
\begin{eqnarray}
\vswfouttlm{\tau}lm\left(k\left(\vect r-\vect r_{1}\right)\right) & = & \begin{cases}
\sum_{\tau'l'm'}\trops_{\tau lm;\tau'l'm'}\left(k\left(\vect r_{2}-\vect r_{1}\right)\right)\vswfouttlm{\tau'}{l'}{m'}\left(\vect r-\vect r_{2}\right), & \vect r\in\openball{\left\Vert \vect r_{2}-\vect r_{1}\right\Vert }{\vect r_{1}}\\
\sum_{\tau'l'm'}\tropr_{\tau lm;\tau'l'm'}\left(k\left(\vect r_{2}-\vect r_{1}\right)\right)\vswfrtlm{\tau'}{l'}{m'}\left(\vect r-\vect r_{2}\right), & \vect r\notin\closedball{\left\Vert \vect r_{2}-\vect r_{1}\right\Vert }{\left|\vect r_{1}\right|}
\end{cases},\label{eq:singular vswf translation}
\end{eqnarray}

\end_inset

where the singular translation operator 
\begin_inset Formula $\trops$
\end_inset

 has the same form as 
\begin_inset Formula $\tropr$
\end_inset

 in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:translation operator"

\end_inset

 except the regular spherical Bessel functions 
\begin_inset Formula $j_{l}$
\end_inset

 are replaced with spherical Hankel functions 
\begin_inset Formula $h_{l}^{(1)}$
\end_inset

.
 
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO note about expansion exactly on the sphere.
\end_layout

\end_inset

 
\end_layout

\begin_layout Standard
As MSTMM deals most of the time with the 
\emph on
expansion coefficients
\emph default
 of fields 
\begin_inset Formula $\rcoeffptlm p{\tau}lm,\outcoeffptlm p{\tau}lm$
\end_inset

 in different origins 
\begin_inset Formula $\vect r_{p}$
\end_inset

 rather than with the VSWFs directly, let us write down how 
\emph on
they
\emph default
 transform under translation.
 Let us assume the field can be in terms of regular waves everywhere, and
 expand it in two different origins 
\begin_inset Formula $\vect r_{p},\vect r_{q}$
\end_inset

,
\begin_inset Formula 
\[
\vect E\left(\vect r,\omega\right)=\sum_{\tau,l,m}\rcoeffptlm p{\tau}lm\vswfrtlm{\tau}lm\left(k\left(\vect r-\vect r_{p}\right)\right)=\sum_{\tau',l',m'}\rcoeffptlm q{\tau'}{l'}{m'}\vswfrtlm{\tau}{'l'}{m'}\left(k\left(\vect r-\vect r_{q}\right)\right).
\]

\end_inset

Re-expanding the waves around 
\begin_inset Formula $\vect r_{p}$
\end_inset

 in terms of waves around 
\begin_inset Formula $\vect r_{q}$
\end_inset

 using 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:regular vswf translation"

\end_inset

, 
\begin_inset Formula 
\[
\vect E\left(\vect r,\omega\right)=\sum_{\tau,l,m}\rcoeffptlm p{\tau}lm\sum_{\tau'l'm'}\tropr_{\tau lm;\tau'l'm'}\left(k\left(\vect r_{q}-\vect r_{p}\right)\right)\vswfrtlm{\tau'}{l'}{m'}\left(\vect r-\vect r_{q}\right)
\]

\end_inset

and comparing to the original expansion around 
\begin_inset Formula $\vect r_{q}$
\end_inset

, we obtain
\begin_inset Formula 
\begin{equation}
\rcoeffptlm q{\tau'}{l'}{m'}=\sum_{\tau,l,m}\tropr_{\tau lm;\tau'l'm'}\left(k\left(\vect r_{q}-\vect r_{p}\right)\right)\rcoeffptlm p{\tau}lm.\label{eq:regular vswf coefficient translation}
\end{equation}

\end_inset

For the sake of readability, we introduce a shorthand matrix form for 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:regular vswf coefficient translation"

\end_inset


\begin_inset Formula 
\begin{equation}
\rcoeffp q=\troprp qp\rcoeffp p\label{eq:reqular vswf coefficient vector translation}
\end{equation}

\end_inset

(note the reversed indices; TODO redefine them in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:regular vswf translation"

\end_inset

, 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:singular vswf translation"

\end_inset

? Similarly, if we had only outgoing waves in the original expansion around
 
\begin_inset Formula $\vect r_{p}$
\end_inset

, we would get
\begin_inset Formula 
\begin{equation}
\rcoeffp q=\tropsp qp\outcoeffp p\label{eq:singular to regular vswf coefficient vector translation}
\end{equation}

\end_inset

for the expansion inside the ball 
\begin_inset Formula $\openball{\left\Vert \vect r_{q}-\vect r_{p}\right\Vert }{\vect r_{p}}$
\end_inset

 
\begin_inset Note Note
status open

\begin_layout Plain Layout
CHECKME
\end_layout

\end_inset

 and 
\begin_inset Formula 
\begin{equation}
\outcoeffp q=\troprp qp\outcoeffp p\label{eq:singular to singular vswf coefficient vector translation-1}
\end{equation}

\end_inset

outside.
\end_layout

\begin_layout Standard
In our convention, the regular translation operator can be expressed explicitly
 as (TODO CHECK CAREFULLY FOR POSSIBLE 
\begin_inset Formula $(-1)^{m'}$
\end_inset

 AND SIMILAR FACTORS AND REWRITE IN TERMS OF SPHERICAL HARMONICS)
\begin_inset Note Note
status open

\begin_layout Plain Layout
Teďka jsou tam zkopírovány výrazy pro C a D z Kristenssona, chybějí fase
\end_layout

\end_inset


\begin_inset Formula 
\begin{multline}
\tropr_{\tau lm;\tau l'm'}\left(\vect d\right)=\frac{\left(-1\right)^{m+m'}}{2}\sum_{\lambda=\left|l-l'\right|}^{l+l'}\left(-1\right)^{\frac{l'-l+\lambda}{2}}\left(2\lambda+1\right)\sqrt{\frac{\left(2l+1\right)\left(2l'+1\right)\left(\lambda-\left(m-m'\right)\right)!}{l\left(l+1\right)l'\left(l'+1\right)\left(\lambda+\left(m-m'\right)\right)!}}\times\\
\times\begin{pmatrix}l & l' & \lambda\\
0 & 0 & 0
\end{pmatrix}\begin{pmatrix}l & l' & \lambda\\
m & -m' & m'-m
\end{pmatrix}\left(l\left(l+1\right)+l'\left(l'+1\right)-\lambda\left(\lambda+1\right)\right)\times\\
\times j_{\lambda}\left(d\right)P_{\lambda}^{m-m'}\left(\cos\theta_{\uvec d}\right)e^{i\left(m-m'\right)\phi_{\uvec d}},\\
\tropr_{\tau lm;\tau'l'm'}\left(\vect d\right)=-i\frac{\left(-1\right)^{m+m'}}{2}\sum_{\lambda=\left|l-l'\right|+1}^{l+l'}\left(-1\right)^{\frac{l'-l+\lambda+1}{2}}\left(2\lambda+1\right)\sqrt{\frac{\left(2l+1\right)\left(2l'+1\right)\left(\lambda-\left(m-m'\right)\right)!}{l\left(l+1\right)l'\left(l'+1\right)\left(\lambda+\left(m-m'\right)\right)!}}\times\\
\times\begin{pmatrix}l & l' & \lambda-1\\
0 & 0 & 0
\end{pmatrix}\begin{pmatrix}l & l' & \lambda\\
m & -m' & m'-m
\end{pmatrix}\sqrt{\lambda^{2}-\left(l-l'\right)^{2}}\sqrt{\left(l+l'+1\right)^{2}-\lambda^{2}}\times\\
\times j_{\lambda}\left(d\right)P_{\lambda}^{m-m'}\left(\cos\theta_{\uvec d}\right)e^{i\left(m-m'\right)\phi_{\uvec d}},\qquad\tau\ne\tau'.\label{eq:translation operator}
\end{multline}

\end_inset

The singular operator 
\begin_inset Formula $\trops$
\end_inset

 for re-expanding outgoing waves into regular ones has the same form except
 the regular spherical Bessel functions 
\begin_inset Formula $j_{l}$
\end_inset

 in are replaced with spherical Hankel functions 
\begin_inset Formula $h_{l}^{(1)}=j_{l}+iy_{l}$
\end_inset

.
\end_layout

\begin_layout Standard
In our convention, the regular translation operator is unitary, 
\begin_inset Formula $\left(\tropr_{\tau lm;\tau'l'm'}\left(\vect d\right)\right)^{-1}=\tropr_{\tau lm;\tau'l'm'}\left(-\vect d\right)=\tropr_{\tau'l'm';\tau lm}^{*}\left(\vect d\right)$
\end_inset

,
\begin_inset Note Note
status open

\begin_layout Plain Layout
todo different notation for the complex conjugation without transposition???
\end_layout

\end_inset

 or in the per-particle matrix notation, 
\begin_inset Formula 
\begin{equation}
\troprp qp^{-1}=\troprp pq=\troprp qp^{\dagger}\label{eq:regular translation unitarity}
\end{equation}

\end_inset

.
 Note that truncation at finite multipole degree breaks the unitarity, 
\begin_inset Formula $\truncated{\troprp qp}L^{-1}\ne\truncated{\troprp pq}L=\truncated{\troprp qp^{\dagger}}L$
\end_inset

, which has to be taken into consideration when evaluating quantities such
 as absorption or scattering cross sections.
 Similarly, the full regular operators can be composed
\begin_inset Note Note
status open

\begin_layout Plain Layout
better wording
\end_layout

\end_inset

, 
\begin_inset Formula 
\begin{equation}
\troprp ac=\troprp ab\troprp bc\label{eq:regular translation composition}
\end{equation}

\end_inset

 but truncation breaks this, 
\begin_inset Formula $\truncated{\troprp ac}L\ne\truncated{\troprp ab}L\truncated{\troprp bc}L.$
\end_inset


\end_layout

\begin_layout Subsubsection
Cross-sections (many scatterers)
\end_layout

\begin_layout Standard
For a system of many scatterers, Kristensson 
\begin_inset CommandInset citation
LatexCommand cite
after "sect. 9.2.2"
key "kristensson_scattering_2016"
literal "false"

\end_inset

 derives only the extinction cross section formula.
 Let us re-derive it together with the many-particle scattering and absorption
 cross sections.
 First, let us take a ball circumscribing all the scatterers at once, 
\begin_inset Formula $\openball R{\vect r_{\square}}\supset\particle$
\end_inset

.
 Outside 
\begin_inset Formula $\openball R{\vect r_{\square}}$
\end_inset

, we can describe the EM fields as if there was only a single scatterer,
\begin_inset Formula 
\[
\vect E\left(\vect r\right)=\sum_{\tau,l,m}\left(\rcoeffptlm{\square}{\tau}lm\vswfrtlm{\tau}lm\left(\vect r-\vect r_{\square}\right)+\outcoeffptlm{\square}{\tau}lm\vswfouttlm{\tau}lm\left(\vect r-\vect r_{\square}\right)\right),
\]

\end_inset

where 
\begin_inset Formula $\rcoeffp{\square},\outcoeffp{\square}$
\end_inset

 are the vectors of VSWF expansion coefficients of the incident and total
 scattered fields, respectively, at origin 
\begin_inset Formula $\vect r_{\square}$
\end_inset

.
 In principle, one could evaluate 
\begin_inset Formula $\outcoeffp{\square}$
\end_inset

 using the translation operators (REF!!!) and use the single-scatterer formulae
 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:extincion CS single"

\end_inset

–
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:absorption CS single"

\end_inset

 with 
\begin_inset Formula $\rcoeffp{}=\rcoeffp{\square},\outcoeffp{}=\outcoeffp{\square}$
\end_inset

 to obtain the cross sections.
 However, this is not suitable for numerical evaluation with truncation
 in multipole degree; hence we need to express them in terms of particle-wise
 expansions 
\begin_inset Formula $\rcoeffp p,\outcoeffp p$
\end_inset

.
 The original incident field re-expanded around 
\begin_inset Formula $p$
\end_inset

-th particle reads according to 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:regular vswf translation"
plural "false"
caps "false"
noprefix "false"

\end_inset


\begin_inset Formula 
\begin{equation}
\rcoeffincp p=\troprp p{\square}\rcoeffp{\square}\label{eq:a_inc local from global}
\end{equation}

\end_inset

whereas the contributions of fields scattered from each particle expanded
 around the global origin 
\begin_inset Formula $\vect r_{\square}$
\end_inset

 is, according to 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:singular vswf translation"
plural "false"
caps "false"
noprefix "false"

\end_inset

,
\begin_inset Formula 
\begin{equation}
\outcoeffp{\square}=\sum_{q\in\mathcal{P}}\troprp{\square}q\outcoeffp q.\label{eq:f global from local}
\end{equation}

\end_inset

Using the unitarity 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:regular translation unitarity"
plural "false"
caps "false"
noprefix "false"

\end_inset

 and composition 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:regular translation composition"
plural "false"
caps "false"
noprefix "false"

\end_inset

 properties, one has
\begin_inset Formula 
\begin{align}
\rcoeffp{\square}^{\dagger}\outcoeffp{\square} & =\rcoeffincp p^{\dagger}\troprp p{\square}\troprp{\square}q\outcoeffp q=\rcoeffincp p^{\dagger}\sum_{q\in\mathcal{P}}\troprp pqf_{q}\nonumber \\
 & =\sum_{q\in\mathcal{P}}\left(\troprp qp\rcoeffincp p\right)^{\dagger}f_{q}=\sum_{q\in\mathcal{P}}\rcoeffincp q^{\dagger}f_{q},\label{eq:atf form multiparticle}
\end{align}

\end_inset

where only the last expression is suitable for numerical evaluation with
 truncated matrices, because the previous ones contain a translation operator
 right next to an incident field coefficient vector (see Sec.
 TODO).
 Similarly, 
\begin_inset Formula 
\begin{align}
\left\Vert \outcoeffp{\square}\right\Vert ^{2} & =\outcoeffp{\square}^{\dagger}\outcoeffp{\square}=\sum_{p\in\mathcal{P}}\left(\troprp{\square}p\outcoeffp p\right)^{\dagger}\sum_{q\in\mathcal{P}}\troprp{\square}q\outcoeffp q\nonumber \\
 & =\sum_{p\in\mathcal{P}}\sum_{q\in\mathcal{P}}\outcoeffp p^{\dagger}\troprp pq\outcoeffp q.\label{eq:f squared form multiparticle}
\end{align}

\end_inset

Substituting 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:atf form multiparticle"
plural "false"
caps "false"
noprefix "false"

\end_inset

, 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:f squared form multiparticle"
plural "false"
caps "false"
noprefix "false"

\end_inset

 into 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:scattering CS single"
plural "false"
caps "false"
noprefix "false"

\end_inset

 and 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:absorption CS single"
plural "false"
caps "false"
noprefix "false"

\end_inset

, we get the many-particle expressions for extinction, scattering and absorption
 cross sections suitable for numerical evaluation: 
\begin_inset Formula 
\begin{eqnarray}
\sigma_{\mathrm{ext}}\left(\uvec k\right) & = & -\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\Re\sum_{p\in\mathcal{P}}\rcoeffincp p^{\dagger}\outcoeffp p=-\frac{1}{2k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\Re\sum_{p\in\mathcal{P}}\rcoeffincp p^{\dagger}\left(\Tp p+\Tp p^{\dagger}\right)\rcoeffp p,\label{eq:extincion CS multi}\\
\sigma_{\mathrm{scat}}\left(\uvec k\right) & = & \frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\sum_{q\in\mathcal{P}}\outcoeffp p^{\dagger}\troprp pq\outcoeffp q\nonumber \\
 &  & =\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\sum_{q\in\mathcal{P}}\rcoeffp p^{\dagger}\Tp p^{\dagger}\troprp pq\Tp q\rcoeffp q,\label{eq:scattering CS multi}\\
\sigma_{\mathrm{abs}}\left(\uvec k\right) & = & -\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\Re\left(\outcoeffp p^{\dagger}\left(\rcoeffincp p+\sum_{q\in\mathcal{P}}\troprp pq\outcoeffp q\right)\right).\nonumber \\
\label{eq:absorption CS multi}
\end{eqnarray}

\end_inset


\begin_inset Note Note
status open

\begin_layout Plain Layout
\begin_inset Formula $=\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\mbox{TODO}.$
\end_inset


\end_layout

\end_inset

An alternative approach to derive the absorption cross section is via a
 power transport argument.
 Note the direct proportionality between absorption cross section 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:absorption CS single"
plural "false"
caps "false"
noprefix "false"

\end_inset

 and net radiated power for single scatterer 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Power transport"
plural "false"
caps "false"
noprefix "false"

\end_inset

, 
\begin_inset Formula $\sigma_{\mathrm{abs}}=-\eta_{0}\eta P/2\left\Vert \vect E_{0}\right\Vert ^{2}$
\end_inset

.
 In the many-particle setup (with non-lossy background medium, so that only
 the particles absorb), the total absorbed power is equal to the sum of
 absorbed powers on each particle, 
\begin_inset Formula $-P=\sum_{p\in\mathcal{P}}-P_{p}$
\end_inset

.
 Using the power transport formula 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:Power transport"
plural "false"
caps "false"
noprefix "false"

\end_inset

 particle-wise gives
\begin_inset Formula 
\begin{equation}
\sigma_{\mathrm{abs}}\left(\uvec k\right)=-\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\left(\Re\left(\rcoeffp p^{\dagger}\outcoeffp p\right)+\left\Vert \outcoeffp p\right\Vert ^{2}\right)\label{eq:absorption CS multi alternative}
\end{equation}

\end_inset

which seems different from 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:absorption CS multi"
plural "false"
caps "false"
noprefix "false"

\end_inset

, but using 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:particle total incident field coefficient a"
plural "false"
caps "false"
noprefix "false"

\end_inset

, we can rewrite it as
\begin_inset Formula 
\begin{align*}
\sigma_{\mathrm{abs}}\left(\uvec k\right) & =-\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\Re\left(\outcoeffp p^{\dagger}\left(\rcoeffp p+\outcoeffp p\right)\right)\\
 & =-\frac{1}{k^{2}\left\Vert \vect E_{0}\right\Vert ^{2}}\sum_{p\in\mathcal{P}}\Re\left(\outcoeffp p^{\dagger}\left(\rcoeffincp p+\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\tropsp pq\outcoeffp q+\outcoeffp p\right)\right).
\end{align*}

\end_inset

It is easy to show that all the terms of 
\begin_inset Formula $\sum_{p\in\mathcal{P}}\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\outcoeffp p^{\dagger}\tropsp pq\outcoeffp q$
\end_inset

 containing the singular spherical Bessel functions 
\begin_inset Formula $y_{l}$
\end_inset

 are imaginary, 
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO better formulation
\end_layout

\end_inset

 so that actually 
\begin_inset Formula $\sum_{p\in\mathcal{P}}\Re\left(\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\outcoeffp p^{\dagger}\tropsp pq\outcoeffp q+\outcoeffp p^{\dagger}\outcoeffp p\right)=\sum_{p\in\mathcal{P}}\Re\left(\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\outcoeffp p^{\dagger}\troprp pq\outcoeffp q+\outcoeffp p^{\dagger}\outcoeffp p\right)=\sum_{p\in\mathcal{P}}\Re\left(\sum_{q\in\mathcal{P}\backslash\left\{ p\right\} }\outcoeffp p^{\dagger}\troprp pq\outcoeffp q+\outcoeffp p^{\dagger}\troprp pp\outcoeffp p\right)=\sum_{p\in\mathcal{P}}\sum_{q\in\mathcal{P}}\outcoeffp p^{\dagger}\troprp pq\outcoeffp q,$
\end_inset

 proving that the expressions in 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:absorption CS multi"
plural "false"
caps "false"
noprefix "false"

\end_inset

 and 
\begin_inset CommandInset ref
LatexCommand eqref
reference "eq:absorption CS multi alternative"
plural "false"
caps "false"
noprefix "false"

\end_inset

 are equal.
\end_layout

\end_body
\end_document
