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%I #22 Oct 31 2013 12:17:23
%S 1,3,21,35,1085,9765,1240155,21082635,1539032355,16929355905,
%T 34654391537535,150169029995985,1230034524697113135,
%U 17630494853991954935,2662204722952785195185,410511968279319477097527
%N Denominators of successive partial sums of sum(1/(2^n-1)).
%H Robert Israel, <a href="/A087690/b087690.txt">Table of n, a(n) for n = 1..103</a>
%F a(n) = a(n-1) (2^n - 1)/gcd(a(n-1) (2^n-1), (2^n-1) A087689(n-1) + a(n-1)). - _Robert Israel_, Dec 28 2012
%e a(4)=35 because 1/1 + 1/3 + 1/7 + 1/15 = 54/35.
%p a:= n -> denom(add(1/(2^i-1),i=1..n)); // _Robert Israel_, Dec 28 2012
%t f[n_] := Denominator[Sum[1/(2^i - 1), {i, n}]]; Array[f, 16] (* _Robert G. Wilson v_, May 25 2011 *)
%t Denominator[Accumulate[1/(2^Range[20]-1)]] (* _Harvey P. Dale_, Dec 28 2012 *)
%Y Cf. A000225, A087689.
%K frac,nonn
%O 1,2
%A _Keenan Pepper_, Sep 27 2003
%E More terms from _Ray Chandler_, Oct 26 2003