Monday, 7 October 2013

logarithms - Series involving log suminftyn=1left(nlogleft(frac2n+12n1right)1right)=frac1log22




Does anybody know how to prove this series?



n=1(nlog(2n+12n1)1)=1log22



I arrived at this through Mathematica.



I tried writing log(2n+12n1) as 1012n12+xdx and k=1(2)kk(2n1)k but none of them worked.


Answer



Note that




n=1(nlog(2n+12n1)1)=limNNn=1(nlog(2n+12n1)1)=limNlog[eNNn=1(2n+12n1)n]=limNlog[eN2NN!(2N+1)N(2N)!].



By Stirling's formula, it follows that




eN2NN!(2N+1)N(2N)!e2.



This immediately yields the desired answer.


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