PROBLEMS WITH ZETA FUNCTION IN SCIPY
Dabbling with zeta in scipy gave various unsatisfactory results. Few very obvious results of zeta function are ;
(1) For negative even integers the function is zero
(2) zeta(0) = -1/2
(3) zeta(-1) = -1/12
(4) zeta(1/2) = -1.46035450880.....
(5) zeta(1) = infinity
(6) zeta(2) = 1.6449340.....
Trying it on scipy,
The probable reason for this inconsistent behaviour and issues for zeta values below 2 and improbable results for negative numbers is; the module is probably structured abinitio from zeta function
whilst it may be a better idea to structure the zeta function using the functional equation.
It may also help to have a special provision for zeta(1/2)
8 comments:
It looks like mpmath gets them correct:
sage: mpmath.zeta(0.0r)
mpf('-0.5')
sage: mpmath.zeta(-1.0r)
mpf('-0.083333333333333329')
sage: mpmath.zeta(0.5r)
mpf('-1.4603545088095868')
sage: mpmath.zeta(1.0r)
Traceback (most recent call last)
ValueError: zeta(1) pole
sage: mpmath.zeta(2.0r)
mpf('1.6449340668482264')
Try using zetac(x) + 1, instead of zeta(x,q). zeta(x,q) returns nan if x < 1.
Also, use 0.5 instead of 1/2, since 1/2 is zero (unless you import division from the future).
Thanks Mike ! ..... mpmath looks great ! .... just that I have not been using it that often ! ..... mpmath is more like MATLAB !
And Sage doesn't do too bad either (I think it uses MPFR behind the scenes):
sage: zeta(0.0)
-0.500000000000000
sage: zeta(-1)
-0.0833333333333333
sage: zeta(0.5)
-1.46035450880959
sage: zeta(1.0)
+infinity
sage: zeta(2.0)
1.64493406684823
Guys !.... many thanks...
mpmath, sage, zetac all work good but I guess that this module zeta(x,q) still needs some improvement !
Today is documentation indisposed, isn't it?
Probably ! it has been a while that I worked with zeta or scipy ! It was a glaring error, the scipy community would have corrected it.
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