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 A271393 #11 Apr 09 2016 15:48:12 %S A271393 3,13,797161 %N A271393 a(1) = 3, a(n+1) = (3^a(n)-1)/2. %C A271393 The next term is too large to include. %C A271393 The terms a(1), a(2), and a(3) are primes. %C A271393 The next term a(4) is a composite number, a pseudoprime to base 3. %C A271393 If a(n) is a pseudoprime to base 3, then a(n+1) is a pseudoprime to base 3. %C A271393 Note that a(n) divides a(n+1)-1 for every n. %C A271393 a(4) has 380343 digits. - _Altug Alkan_, Apr 09 2016 %D A271393 R. Steuerwald (1948), see A005935. %t A271393 NestList[(3^# - 1)/2 &, 3, 3] (* _Michael De Vlieger_, Apr 06 2016 *) %Y A271393 Cf. A005935 (see Steuerwald's theorem), A007013, A028491. %K A271393 nonn %O A271393 1,1 %A A271393 _Thomas Ordowski_, Apr 06 2016