Compute the cross-power spectral density of two real functions for a single spherical harmonic degree.

## Usage

cpsd = SHCrossPowerDensityL (cilm1, cilm2, l)

## Parameters

cpsd : output, real(dp)
Cross-power spectral density of the two real functions for spherical harmonic degree l.
cilm1 : input, real(dp), dimension (2, lmaxin1+1, lmaxin1+1)
The spherical harmonic coefficients of the first real function.
cilm2 : input, real(dp), dimension (2, lmaxin2+1, lmaxin2+1)
The spherical harmonic coefficients of the second real function.
l : input, integer
The spherical harmonic degree. This must be less than or equal to the minimum of lmaxin1 and lmaxin2.

## Description

SHCrossPowerDensityL will calculate the cross-power spectral density of two real functions expressed in real 4-pi normalized spherical harmonics for a single spherical harmonic degree l. This is explicitly calculated as:

cpsd = Sum_{i=1}^2 Sum_{m=0}^l cilm1(i, l+1, m+1) * cilm2(i, l+1, m+1) / (2l + 1).