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 A139307 #8 Oct 17 2024 08:26:21 %S A139307 7,31,511,8191,33554431,8589934591,137438953471,2305843009213693951, %T A139307 2658455991569831745807614120560689151, %U A139307 191561942608236107294793378393788647952342390272950271 %N A139307 a(n) = (2^(2*p - 1)) - 1, where p is A000043(n). %C A139307 Ultraperfect numbers (A139306) minus 1. %H A139307 Amiram Eldar, <a href="/A139307/b139307.txt">Table of n, a(n) for n = 1..15</a> %F A139307 a(n) = (2^(2*A000043(n) - 1)) - 1 = A139306(n) - 1. %e A139307 a(5) = 33554431 because A000043(5) = 13 and (2^(2*13 - 1))-1 = 2^25 - 1 = 33554431. %t A139307 2^(2 * MersennePrimeExponent[Range[10]] - 1) - 1 (* _Amiram Eldar_, Oct 17 2024 *) %Y A139307 Cf. A000043, A139287, A139295, A139306. %K A139307 nonn %O A139307 1,1 %A A139307 _Omar E. Pol_, Apr 13 2008, May 08 2008