Incorrect result from gegenbauer function of order 0 #494
Description
mpmath version: 1.1.0
Though the gegenbauer polynomial of degree zero is defined to be 1 over its whole domain, we get a result of 0.0 when the second argument is 0. This is true in both the fp and mp contexts.
Example:
Python 3.6.8 (default, Aug 20 2019, 17:12:48)
[GCC 8.3.0] on linux
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>>> from mpmath import *
>>> mp.gegenbauer(0, 0, 2.2)
mpf('0.0')
>>> mp.gegenbauer(0, 1, 2.2)
mpf('1.0')
>>> mp.gegenbauer(0, 4, 2.2)
mpf('1.0')
>>> mp.gegenbauer(0, 0, 1.8)
mpf('0.0')
>>> mp.gegenbauer(0, 1, 1.8)
mpf('1.0')