This routine returns the spherical harmonic coupling matrix for a given set of spherical-cap Slepian basis functions. This matrix relates the power spectrum expectation of the function expressed in a subset of the best-localized Slepian functions to the expectation of the global power spectrum.

## Usage

kij = SHSCouplingMatrixCap (galpha, galpha_order, nmax)

## Returns

kij : float, dimension (lmax+1, lmax+1)
The coupling matrix that relates the power spectrum expectation of the function expressed in a subset of the best-localized spherical-cap Slepian functions to the expectation of the global power spectrum.

## Parameters

galpha : float, dimension (lmax+1, nmax)
An array of spherical-cap Slepian functions, arranged in columns from best to worst localized.
galpha_order : integer, dimension (kmaxin)
The angular order of the spherical-cap Slepian functions in galpha.
nmax : input, integer
The number of Slepian functions used in reconstructing the function.

## Description

SHSCouplingMatrixCap returns the spherical harmonic coupling matrix that relates the power spectrum expectation of the function expressed in a subset of the best-localized spherical-cap Slepian functions to the expectation of the global power spectrum (assumed to be stationary). The Slepian functions are determined by a call to SHReturnTapers and each row of galpha contains the (lmax+1) spherical harmonic coefficients for the single angular order as given in galpha_order.

The relationship between the global and localized power spectra is given by:

< S_{\tilde{f}}(l) > = \sum_{l'=0}^lmax K_{ll'} S_{f}(l')

where S_{\tilde{f}} is the expectation of the power spectrum at degree l of the function expressed in Slepian functions, S_{f}(l’) is the expectation of the global power spectrum, and < … > is the expectation operator. The coupling matrix is given explicitly by

K_{ll'} = \frac{1}{2l'+1} Sum_{m=-mmax}^mmax ( Sum_{alpha=1}^nmax g_{l'm}(alpha) g_{lm}(alpha) )**2

where mmax is min(l, l’).

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