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 A091421 #23 Aug 31 2024 04:33:33
%S A091421 1,2,3,4,6,7,8,15,19,31,68,69,78,82,162,210,524,770,1058,1437,1730,
%T A091421 3977,5104,8440
%N A091421 Numbers m such that m#*2^m - 1 is prime, where m# = A002110(m).
%C A091421 1# = 2 2# = 2*3 = 6 3# = 2*3*5 = 30.
%C A091421 No more terms < 5000. - _L. Joris Perrenet_, Mar 17 2020
%e A091421 a(1) = 1 because 1#*2^1 - 1 = 3 is prime.
%e A091421 a(2) = 2 because 2#*2^2 - 1 = 23 is prime.
%t A091421 For[n = 1, n < 60, n++, If[PrimeQ[2^n*Product[Prime[i], {i, 1, n}] - 1], Print[n]]] (* _Stefan Steinerberger_, Feb 06 2006 *)
%o A091421 (PARI) pp(n)= s=1; for(i=1,n,s=s*prime(i)); return(s);
%o A091421 f(n)=pp(n)!*2^n -1;
%o A091421 for (i=1,500,if(isprime(f(i)),print1(i, ", ")))
%Y A091421 Cf. A002110, A091424.
%K A091421 nonn,hard,more
%O A091421 1,2
%A A091421 Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004
%E A091421 a(17) from _Stefan Steinerberger_, Feb 06 2006
%E A091421 a(18)-a(22) from _L. Joris Perrenet_, Mar 17 2020
%E A091421 a(23)-a(24) from _Michael S. Branicky_, Aug 28 2024