%I
%S 2,5,41,21523361,926510094425921,1716841910146256242328924544641
%N Primes of the form (3^n+1)/2.
%C Primes can only occur for n=2^k. The next term (if it exists) has k>=21 (i.e. more than 10^6 decimal digits).  _Don Reble_, Apr 28 2004. [For the latest results about the search, see A171381.  _N. J. A. Sloane_, Jun 14 2010]
%e a(3)=41 because it can be written as (3^4+1)/2=82/2.
%Y A171381 is the main entry for this sequence. Cf. also A138083.
%K nonn,hard
%O 1,1
%A _Hugo Pfoertner_, Apr 19 2004
%E Erroneous terms deleted by _N. J. A. Sloane_, Jun 14 2010
