[WIP] Fix digamma integral #26563
[WIP] Fix digamma integral #26563
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| def _eval_is_finite(self): | ||
| z, s, a = self.args | ||
| if z == 0 or a == 0: | ||
| return z.is_zero and a.is_zero and s.is_nonpositive |
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What should happen here if any of the is_ checks gives None?
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Hi @oscarbenjamin , thanks for taking a look and the information about the booleans.
I thought python and was compatible with the fuzzy logic but it is indeed not. I will use the fuzzy_and function, this should handle correctly the None cases.
Also, I am not sure what is the convention when the function evaluates to NaN, should the is_finite be None?
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what is the convention when the function evaluates to NaN, should the is_finite be None?
Probably. What case evaluates to nan that you are thinking of?
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It is because a lot of situations where the function is infinite is because one has s.is_nonpositive gives False when s is not real. I have added a check for s to be real, when s is complex the expression is undefined so now the function should return None.
Fixed some edge cases. Check that s is real for 0**s. Return None when the arguments are infinite.
| exp_expr = self.expand(func=True) | ||
| if exp_expr != self: | ||
| return exp_expr.is_finite |
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What is expand(func=True) being used for here?
We should avoid creating new expressions during an assumptions query. Just creating the new expression runs the risk of triggering an identical assumptions query which leads to infinite recursion.
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Ok, I did not think about that. My idea was that, since LerchPhi includes a lot of common functions as particular values (zeta function, polylog, Dirichlet eta, ...), I can try to write the function in terms of these other functions and then let them determine whether the function is finite or not. Is there a safe way to do it without the risks you are mentioning?
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There is not really a safe way to do it. We just have to accept that the core assumptions cannot resolve all queries:
https://docs.sympy.org/latest/guides/assumptions.html
| if a.is_zero is False: | ||
| return True | ||
| if s.is_real: | ||
| return s.is_nonpositive |
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I'm not sure if this works correctly when a.is_zero is None.
References to other Issues or PRs
Fixes #26523
Brief description of what is fixed or changed
Added regression tests
Other comments
Release Notes