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