A087690 Denominators of successive partial sums of sum(1/(2^n-1)).

1, 3, 21, 35, 1085, 9765, 1240155, 21082635, 1539032355, 16929355905, 34654391537535, 150169029995985, 1230034524697113135, 17630494853991954935, 2662204722952785195185, 410511968279319477097527

Offset

1,2

Links

Robert Israel, Table of n, a(n) for n = 1..103

Formula

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

Example

a(4)=35 because 1/1 + 1/3 + 1/7 + 1/15 = 54/35.

Maple

a:= n -> denom(add(1/(2^i-1), i=1..n)); // Robert Israel, Dec 28 2012

Mathematica

f[n_] := Denominator[Sum[1/(2^i - 1), {i, n}]]; Array[f, 16] (* Robert G. Wilson v, May 25 2011 *)
Denominator[Accumulate[1/(2^Range[20]-1)]] (* Harvey P. Dale, Dec 28 2012 *)

Crossrefs

Cf. A000225, A087689.

Sequence in context: A100986 A213141 A075732 * A191763 A227241 A076169

Adjacent sequences: A087687 A087688 A087689 * A087691 A087692 A087693

Keyword

frac,nonn

Author

Keenan Pepper, Sep 27 2003

Extensions

More terms from Ray Chandler, Oct 26 2003

Status

approved