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 A101050 #13 Aug 12 2025 02:18:18
%S A101050 1,0,2,1,2,3,2,1,4,4,1,1,2,7,4,2,12,3,5,2,7,1,2,4,1,10,3,10,9,8,25,2,
%T A101050 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 A101050 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 A101050 Least k such that prime(n)*2^k-1 is prime, or -1 if no such k exists.
%C A101050 Primes p such that p*2^k-1 is composite for all k are called Riesel numbers. The smallest known Riesel number is the prime 509203. Currently, 2293 is the smallest prime whose status is unknown. For a(120), which corresponds to the prime 659, Dave Linton found the least k is 800516. - _T. D. Noe_, Aug 04 2005
%D A101050 See A046069
%t A101050 Table[p=Prime[n]; k=0; While[ !PrimeQ[ -1+p*2^k], k++ ]; k, {n, 119}] (* _T. D. Noe_, Aug 04 2005 *)
%Y A101050 Cf. A046069 (least k such that (2n-1)*2^k-1 is prime).
%K A101050 nonn
%O A101050 1,3
%A A101050 _Pierre CAMI_, Jan 21 2005
%E A101050 Corrected and extended by _T. D. Noe_, Aug 04 2005