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%I #2 Mar 30 2012 17:23:26
%S 1,6,27,405,1458,5103,59049,196830,649539,6908733,22320522,71744535,
%T 731794257,2324522934,7360989291,73222472421,230127770466,
%U 721764371007,7060738412025,22029503845518,68630377364883,663426981193869
%N Denominators of ternary BBP-type series for log(5)
%C The formula
%C log(5)=(4/27)Sum(k>=0,(1/81^k)(9/(4k+1)+3/(4k+2)+1/(4k+3)))
%C can be written in unit numerators:
%C log(5)=(4/3)Sum(k>=0,(1/81^k)(1/(4k+1)+1/(3*(4k+2))+1/(9*(4k+3)))),
%C so the sequence of denominators inside the sum satisfies
%C Sum(n>=0,1/a(n))=(3/4)log(5)
%F G.f.: (1+6*x+27*x^2+243*x^3+486*x^4+729*x^5)/(1-81*x^3)^2
%o (PARI) a(n)=[(n\3*4+1),3*(n\3*4+2),9*(n\3*4+3)][n%3+1]*81^(n\3)
%Y Cf. A154920.
%K frac,nonn
%O 0,2
%A _Jaume Oliver Lafont_, Sep 03 2009
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