PlmIndex.f95

integer function PlmIndex(l, m)
!-------------------------------------------------------------------------------
!
!   This function will return the index corresponding 
!   to a given l and m in the arrays of Legendre Polynomials
!   generated by routines such as PlmBar and PlmSchmidt.
!
!   Calling Parameters
!
!       l   Spherical harmonic angular degree.
!       m   Spherical harmonic angular order.
!
!   Copyright (c) 2005-2019, SHTOOLS
!   All rights reserved.
!
!-------------------------------------------------------------------------------
    implicit none

    integer, intent(in) :: l, m

    if (l < 0) then
        print*, "Error --- PlmIndex"
        print*, "L must be greater of equal to 0."
        print*, "L = ", l
        print*, "M = ", m
        stop

    else if (m < 0 .or.  m > l) then
        print*, "Error --- PlmIndex"
        print*, "M must be greater than or equal to zero and less than or " // &
                "equal to L."
        print*, "L = ", l
        print*, "M = ", m
        stop

    end if

    PlmIndex = (l*(l+1))/2+m+1

end function PlmIndex