Find the sum of the following series:
1 + 4/7 + 9/49 + 16/343 + .......... to infinity
(In reply to
re: solution  seems to be wrong by Charlie)
The correct answer, I believe, is 392/216=49/27=1.814814814... . I base this on the formula
Sum_{n=1..infinity} n^2*x^n=x*(x+1)/(1x)^3
for the generating function of n^2 taken from Weisstein's online
article "Generating Function." This result can be proved by
differentiating the wellknown series
1/(1x)=1+x+x^2+x^3+... twice and doing some arithmetic. These series
all converge absolutely for x<1, so the differentiation is
legitimate. The sum sought is thus 7 times the generating function for
n^2, evaluated at x=1/7.

Posted by Richard
on 20060311 13:03:38 