Search All of the Math Forum:
Views expressed in these public forums are not endorsed by
NCTM or The Math Forum.



Re: Infinite Series Error  Bug?
Posted:
Jan 25, 2012 7:00 AM


Clearly Sum cannot find an explicit formula for the sum of this series. What seems to occur at some stage looks like underflow to me, but why Mathematica attempts any numerical computations in this case and if this is a bug I can't tell. However, Mathematica can tell you two things.
1. That the series is convergent
SumConvergence[q^(6 + 4*n)/(1  q^(5 + 4*n)), n]
q != 0 && Abs[q]^4 < 1
The first condition is curious but it seems right. Secondly, once you know that:
With[{q = 1/10}, NSum[q^(6 + 4*n)/(1  q^(5 + 4*n)), {n, 0, 10^8}]]
21.1012
or, if you don't trust machine precision:
With[{q = 1/10}, NSum[q^(6 + 4*n)/(1  q^(5 + 4*n)), {n, 0, Infinity}, WorkingPrecision > 50]]
21.101200102001001100190002090100091000992090000000
Andrzej Kozlowski
On 24 Jan 2012, at 11:06, clashton wrote:
> Can anyone explain why evaluating the following expression gives an > error? > > Sum[q^(6 + 4*n)/(1  q^(5 + 4*n)), {n, 0, Infinity}] /. {q > 0.1} > > None of the denominators are 0 (would only happen if 5 + 4*n=0, which > is impossible for an integer n, yet I get an error message "Infinite > expression 1/0 encountered" (several times in fact). > > Any help on a work around greatly appreciated. > > (Please reply to my personal email also). > > Thanks, > > Jimmy >



