python - Why can't i raise to a negative power in numpy? -


i'm modelling riemann theta function:

import numpy np def theta(s, n=100):     a_range = np.arange(2, n + 1)     return 1 + sum(1/(a_range ** s))

it not work negative s; e.g. theta(-2) leads error:

      1 def theta(s, n=100):       2     a_range = np.arange(1) ----> 3     return 1 + sum(1/(a_range ** s))       4        5 theta(-2)        valueerror: integers negative integer powers not allowed.

why's that? x^-1 should 1/x if recall math correctly.

in numpy, output dtype of operation a_range ** s determined input dtypes, not values. means raising integers integer powers must give integers, or give floats.

it's important numpy.array([2]) ** numpy.array([2]) give integer output, means numpy.array([2]) ** numpy.array([-2]) has give integers or nothing. picked nothing; raising integers negative integer powers error in numpy.

if want floating-point output, make floating-point input:

a_range = np.arange(2, n + 1, dtype=float)

or

a_range = np.arange(2, n + 1).astype(float)

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