Let there be the following program
from sympy import *
n,m=symbols('n m')
print(binomial(n,m).subs([(m,0),(n,-1)]))
print(binomial(n,m).subs([(n,-1),(m,0)]))
result:
1
zoo
Why is the result different?
Asked
Active
Viewed 64 times
2
Vladimir
- 21
- 1
-
Unrelated with the 'zoo' tag, in sympy zoo is the complex infinity. That said, pretty good question, I can imagine how sympy tried to simplify the first substitution *then* the second, and failed, but it's clearly a bug. The first is correct, with usual definitions of binomial coefficients for negative n. – Sep 26 '21 at 20:52
-
Not sure what the conclusion is but this PR shows a length discussion about binomial coefficients at negative integers: https://github.com/sympy/sympy/pull/18960 – Oscar Benjamin Sep 26 '21 at 21:45
-
1@OscarBenjamin I don't think it's related. The question there is about binomial(n,k) for negative k. For negative n and nonnegative k there is no problem, using the usual binomial(x,k)=x(x-1)...(x-k+1)/k!, for all x (even noninteger). – Sep 26 '21 at 22:13