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How did simplify_full() manipulate this expression?

asked 2019-08-30 10:48:04 -0500

joakim_uhlin gravatar image

This is sort of a follow up to a previous question of mine: https://ask.sagemath.org/question/475...

I wanted to prove a certain combinatorial equality. Now, to prove this identity in Sage, I can define the two function $\text{LHS}(n,k)$ and $\text{RHS}(n,k)$ representing the left- and righthand side and make sure that $\text{LHS}(n,k)-\text{RHS}(n,k)=0$ with the following code (actually the code provides a proof of a slightly more general identity, which holds for other values of $n, k$ as well.):

var('n k')

def LHS(n,k):
    return n/2*(
        (n/2+1-k)/(n/2+1)*((n/2+1)/(k/2)*binomial(n/2-3,k-4)*2**(n/2-k+1)*1/(k/2-1)*binomial(k-4,k/2-2))+
        2*(k/2+1)/(n/2+1)*((n/2+1)/(k/2+1)*binomial(n/2-3,k-2)*2**(n/2-k-1)*1/(k/2)*binomial(k-2,k/2-1)))

def RHS(n,k):
    return (n/k)*(2**(n/2-k))*binomial(n/2-2,k-2)*binomial(k-2,k/2-1)

(LHS(n,k)-RHS(n,k)).simplify_full()

This outputs

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which is what I want but I would like to prove this identity with pen-and-paper and this seems to be a bit tricky. I wonder if there is a way to see how exactly simplify_full() were able simplify the expressions?

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rburing gravatar imagerburing ( 2019-09-05 03:33:54 -0500 )edit

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answered 2019-08-30 11:50:35 -0500

Emmanuel Charpentier gravatar image

updated 2019-09-05 01:36:52 -0500

Well...

sage: (LHS(n,k)-RHS(n,k)).simplify_factorial().canonicalize_radical()
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is a big fat red hint...

EDIT : The big fat red hint is that canonicalize_radical transforms an "horrible`expression to someting more palatable...

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Right. As the expression obtained by writing "(LHS(n,k)-RHS(n,k)).simplify_factorial()" is quite horrible, I suspect that Sage will not be able to give me any good insight into how to prove this equality.

joakim_uhlin gravatar imagejoakim_uhlin ( 2019-08-30 13:05:52 -0500 )edit

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Asked: 2019-08-30 10:48:04 -0500

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Last updated: Sep 05