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I'm trying to do the following sum:

NSum[((50000 - x) choose x)*(1/3^15)^x*((3^15 - x)/3^15)^(50000 - x), {x, 1, 4}]

But I keep getting an error saying:

"Summand (or its derivative) ((50000-x)\ choose\ x)\ (1/3^15)^x\ \((3^15-x)/3^15)^(50000-x) is not numerical at point x = 1."

Does anyone know what might be wrong?

Thanks

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    $\begingroup$ what's choose? $\endgroup$
    – Verbeia
    Dec 7, 2014 at 7:10
  • $\begingroup$ @Verbeia Isn't n choose r C(n, r)? $\endgroup$
    – Kar
    Dec 7, 2014 at 7:13
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    $\begingroup$ Not in Mathematica syntax, it isn't. I think you need Binomial. $\endgroup$
    – Verbeia
    Dec 7, 2014 at 7:14

1 Answer 1

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You are most likely hitting this problem because choose is not defined to be a numerical quantity. NSum requires a summation where each part evaluates to something numerical, not algebraic. Deleting the choose gives a result:

In[1]:= NSum[((50000 - x)  x)*(1/3^15)^
   x*((3^15 - x)/3^15)^(50000 - x), {x, 1, 4}]

Out[1]= 0.0034724

EDIT

Based on the additional information you have provided, you might be looking for Binomial.

NSum[Binomial[(50000 - x) , x]*(1/3^15)^x*((3^15 - x)/3^15)^(50000 - x), {x, 1, 4}]

(* 0.00347843 *)
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