Function fringe_index 

pub const fn fringe_index(n: u32, m: i32) -> u32

The Fringe (Air Force / Arizona) single index of mode (n, m), from 1.

$j = (1 + (n + |m|)/2)^2 - 2|m| + [m < 0]$. Ordering is by spatial frequency $n + |m|$ rather than by radial degree, which is why Fringe truncations (the classic 37-term set) keep low-frequency high-degree terms that an ANSI truncation at the same count would drop.