This IDL routine provides a means to convert the a_lm index i=l² + l+ m + 1 (as returned by eg the fits2alm routine) into l and m.

Location in HEALPix directory tree: src/idl/misc/index2lm.pro

FORMAT

index: Long array containing on INPUT the index
i = l² + l + m + 1.
l: Long array containing on OUTPUT the order l. It has the same size as index.
m: Long array containing on OUTPUT the degree m. It has the same size as index.

index2lm converts i=l² + l+ m + 1 into (l, m). Note that the index i is only defined for $0 \le \vert m\vert\le l$ .

idl: version 6.1 or more is necessary to run index2lm.
fits2alm: reads a FITS file containing a_lm values.
alm2fits: writes a_lm values into a FITS file.
lm2index: routine complementary to index2lm: converts (l, m) into i = l² + l + m + 1.

index2lm, index, l, m

will return in l and m the order l and degree m such that index =l² + ell + m + 1

Eric Hivon 2010-06-18

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