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 A139291 #24 Oct 10 2021 22:45:03 %S A139291 1,7,127,2047,524287,8388607,2147483647,34359738367,8796093022207, %T A139291 36028797018963967,576460752303423487,2361183241434822606847, %U A139291 604462909807314587353087,9671406556917033397649407,2475880078570760549798248447,10141204801825835211973625643007 %N A139291 a(n) = 2^(2*prime(n) - 3) - 1. %F A139291 a(n) = A139286(n)/4 - 1. %F A139291 a(n) = A000225(A131426(n)). - _Max Alekseyev_, Mar 07 2020 %F A139291 a(n) = A139290(n) - 1. - _Omar E. Pol_, Mar 10 2020 %t A139291 (2^(2#-1))/4-1&/@Prime[Range[20]] (* _Harvey P. Dale_, May 08 2011 *) %Y A139291 Cf. A139286, A139287, A139288, A139289, A139290, A139293, A139294, A139306. %K A139291 nonn %O A139291 1,2 %A A139291 _Omar E. Pol_, Apr 13 2008 %E A139291 Edited by _Max Alekseyev_, May 03 2009, Mar 07 2020