wrong subs in rising_factorial

[mantepse]

Steps To Reproduce

sage: var("a, b")
(a, b)
sage: rising_factorial(a, b).subs({a:1})
gamma(b + 1)
sage: rising_factorial(-a, b).subs({a:1})
0

Expected Behavior

The result should be rising_factorial(-1, b), which is 0 only if b > 1.

Actual Behavior

The result is 0.

Additional Information

#19461

Environment

not relevant

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[harith-hacky03]

@mantepse Can start working on this?

[mantepse]

No need to ask, @harith-hacky03. If you think you can do it, propose a pull request. Do you understand the mathematics behind it?

[pawani27]

@mantepse Can this be happening because gamma(-1) is infinity? To solve this can we just put a condition there that is: if a<0 and b>(-a) only then for gamma to return?
I am a new contributor, so I apologise if it sounds stupid

[harith-hacky03]

@mantepse Yes , i understand it , i got some conclusions can you let me know is it correct or anything needs to be changed ?

Thank you : )