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 A139294 #17 Jul 10 2025 16:02:40
%S A139294 32,8192,2305843009213693952,
%T A139294 14474011154664524427946373126085988481658748083205070504932198000989141204992
%N A139294 a(n) = 2^(2p - 1), where p is the n-th Mersenne prime A000668(n).
%C A139294 Next terms have 4932, 78913, 315652, 1292913986, and 1388255822130839283 decimal digits. - _Jens Kruse Andersen_, Jul 14 2014
%F A139294 a(n) = 2^(2*A000668(n)-1).
%t A139294 A000668 := Select[2^Range[500] - 1, PrimeQ]; Table[2^(2*A000668[[n]] - 1), {n, 1, 5}] (* _G. C. Greubel_, Oct 03 2017 *)
%t A139294 a[n_] := 2^(2^(MersennePrimeExponent[n] + 1) - 3); Array[a, 4] (* _Amiram Eldar_, Jul 10 2025 *)
%o A139294 (PARI) \p 100
%o A139294 print1("a(n): "); forprime(q=2, 7, p=2^q-1; if(isprime(p), print1(2^(2*p-1)", ")));
%o A139294 print1("\nNumber of digits in a(n): "); forprime(q=2, 127, p=2^q-1; if(isprime(p), print1(ceil((2*p-1)*log(2)/log(10))", "))) \\ _Jens Kruse Andersen_, Jul 14 2014
%Y A139294 Cf. A000079, A000668, A139286, A139295, A139296, A139297, A139298, A139299, A139300, A139301, A139302, A139303, A139304, A139305, A139306.
%K A139294 nonn
%O A139294 1,1
%A A139294 _Omar E. Pol_, Apr 13 2008