# healpy.sphtfunc.Alm.getidx¶

static Alm.getidx(lmax, l, m)

Returns index corresponding to (l,m) in an array describing alm up to lmax.

In HEALPix C++ and healpy, $$a_{lm}$$ coefficients are stored ordered by $$m$$. I.e. if $$\ell_{max}$$ is 16, the first 16 elements are $$m=0, \ell=0-16$$, then the following 15 elements are $$m=1, \ell=1-16$$, then $$m=2, \ell=2-16$$ and so on until the last element, the 153th, is $$m=16, \ell=16$$.

Parameters
lmaxint

The maximum l, defines the alm layout

lint

The l for which to get the index

mint

The m for which to get the index

Returns
idxint

The index corresponding to (l,m)