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psum(data, powers [, axes, class, /onedim,
/vnormalize, /cnormalize, /single]
Returns a weighted sum of data. The weights are equal to
the coordinates in the dimensions given in axes to the
integer powers given in powers. powers and
axes may be scalars or arrays and must have the same number
of elements. If axes is not specified, then
indgen(powers) is assumed. If class is
defined, then it must contain a class number for each element of
data and the results will be gathered for each class
separately. Keyword /onedim indicates that data
should be treated as a 1d array. Keyword /vnormalize forces
normalization with the total of the values for each result, and keyword
/cnormalize with the total of the coordinate weights for each
results. Keyword /single returns a single (weighted) sum - if
axis was not specified. All calculations are performed in
double precision and the result has data type double.
If w = indgen(data, 0) then psum(data, n, 0) is equal to
total(data*w^n,0), and psum(data, n, 1, /cnormalize is
equal to total(data*w^n)/total(w^n).