Compute the index of a array of Legendre functions corresponding to degree l and angular order m.
index = PlmIndex (l, m)
index : integer, ndarray : Index of an array of associated Legendre functions corresponding to degree l and angular order m.
l : integer, array_like : The spherical harmonic degree.
m : integer, array_like : The angular order.
PlmIndex will calculate the index of an array of associated Legendre functions corresponding to degree l and angular order m. The input arrays are generated by routines such as PlmBar, PlmBar_d1, PlmSchmidt, PlmSchmidt_d1, PlmON, PlmON_d1, PLegendreA, and PLegendreA_d1. The array index in Python is equal to l * (l + 1) / 2 + m.