Compute the power spectral density of a real function for a single spherical harmonic degree.

## Usage

psd = SHPowerDensityL (cilm, l)

## Parameters

psd : output, real(dp)
Power spectral density of the function for the spherical harmonic degree l.
cilm : input, real(dp), dimension (2, lmaxin+1, lmaxin+1)
The function expressed in real spherical harmonics.
l : input, integer
The spherical harmonic degree. This must be less than or equal to lmaxin.

## Description

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

psd = Sum_{i=1}^2 Sum_{m=0}^l cilm(i, l+1, m+1)**2 / (2l + 1).