This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A128979 #14 Jan 01 2024 11:11:02
%S A128979 1,1,2,1,2,3,2,1,4,4,1,1,2,7,4,2,12,3,5,2,7,1,2,4,1,10,3,10,9,8,25,2,
%T A128979 2,1,4,5,1,3,4,2,8,3,226,3,2,1,1,3,2,1,4,4,11,6,4,2,8,1,5,2,11,2,1,26,
%U A128979 3,6,1,1,18,3,4,4,1,7,1,2,20,5,10,3,4,7,2,3,1,6,112,9,10,7,2,12,5,46,1,2,8
%N A128979 Least exponent k such that p_n*(2^k) - 1 is prime.
%C A128979 Supposedly the difference from A101050 is that the k here are required to be strictly positive (nonzero positive). - _R. J. Mathar_, Dec 13 2008
%H A128979 Pierre CAMI and Robert G. Wilson v, <a href="/A128979/b128979.txt">Table of n, a(n) for n = 1..119</a>.
%t A128979 f[n_] := Block[{k = 1, p = Prime@n}, While[ !PrimeQ[p*2^k - 1], k++ ]; k]; Array[f, 97]
%Y A128979 Cf. A000040, A101050, A126715.
%K A128979 nonn
%O A128979 1,3
%A A128979 _Pierre CAMI_ and _Robert G. Wilson v_, Feb 16 2007