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 A191619 #18 Jul 22 2025 12:11:13
%S A191619 0,0,3,0,3,10,3,0,3,10,3,4,3,4,3,16,23,4,3,21,12,10,18,40,14,37,8,16,
%T A191619 32,10,36,1,63,10,3,48,17,67,3,31,33,22,9,19,3,9,47,33,21,15,3,58,51,
%U A191619 22,78,163,8,30,3,85,44,4,71,28,204,4,42,75,27,16,17
%N A191619 Least a such that (2^n-a)*2^n + 1 is a prime number.
%C A191619 Does a(n) exist for every n? This does not seem to be known, even on the GRH; see Heath-Brown. [_Charles R Greathouse IV_, Dec 27 2011]
%H A191619 Pierre CAMI, <a href="/A191619/b191619.txt">Table of n, a(n) for n = 1..5000</a>
%H A191619 D. R. Heath-Brown, <a href="https://web.archive.org/web/20160317102203/http://eprints.maths.ox.ac.uk/166/1/linnik.pdf">Zero-free regions for Dirichlet L-functions, and the least prime in an arithmetic progression</a>. Proceedings of the London Mathematical Society 64:3 (1992), pp. 265-338.
%t A191619 Table[a = 0; While[! PrimeQ[(2^n - a)*2^n + 1], a++]; a, {n, 100}] (* _T. D. Noe_, Jun 11 2011 *)
%o A191619 (PARI) a(n)=forstep(k=4^n+1, 1, -2^n, if(ispseudoprime(k), return(2^n-(k-1)>>n))) \\ _Charles R Greathouse IV_, Dec 27 2011
%Y A191619 Cf. A191617, A191618, A191620, A191621.
%K A191619 nonn
%O A191619 1,3
%A A191619 _Pierre CAMI_, Jun 09 2011