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 A339579 #26 Dec 25 2020 10:37:40 %S A339579 4,3,5,2,0,4,0,1,0,0,0,3,0,1,0,0,0,1,0,1,0,0,0,2,0,0,0,0,0,2,0,1,0,0, %T A339579 0,0,0,1,0,0,0,3,0,1,0,0,0,1,0,0,0,0,0,2,0,0,0,0,0,1,0,1,0,0,0,0,0,1, %U A339579 0,0,0,1,0,1,0,0,0,0,0,1,0,0,0,2,0,0,0,0,0,6,0,0,0,0,0 %N A339579 a(n) = least nonnegative integer k such that n*2^k - 1 is composite. %C A339579 Conjectured to grow without limit. %C A339579 A063377 is an essentially identical sequence, although with a slightly different definition, different initial terms, and different offset. %D A339579 Carl Pomerance, Problem 81:21 (= 321), in R. K. Guy problem list. %H A339579 Antti Karttunen, <a href="/A339579/b339579.txt">Table of n, a(n) for n = 1..65537</a> %H A339579 R. K. Guy, editor, <a href="/A339579/a339579.pdf">Western Number Theory Problems, 1985-12-21 & 23</a>, Typescript, Jul 13 1986, Dept. of Math. and Stat., Univ. Calgary, 11 pages. Annotated scan of pages 1, 3, 7, 9, with permission. %F A339579 For n >= 3, a(n) = A063377(n-1). %o A339579 (PARI) A339579(n) = for(k=0,oo,my(t=(n*(2^k))-1); if((t>1)&&!isprime(t), return(k))); \\ _Antti Karttunen_, Dec 24 2020 %Y A339579 See A339580 for records. %Y A339579 Cf. A063377, A063378, A339581. %K A339579 nonn %O A339579 1,1 %A A339579 _N. J. A. Sloane_, Dec 24 2020