%I #16 Sep 08 2022 08:46:14
%S 1,2,5,9,19,25,54,104,112,120,177,317,504,540,734,780,1649,1923,2715,
%T 4308,5917,6494,7305,22653,26888,71448,93834,137027,158472,174648
%N Numbers n such that 16^n  4^n  1 is prime.
%C For n = 1, 2, 5, 9, 19, 25, the corresponding primes are 11, 239, 1047551, 68719214591, 75557863725639445512191, 1267650600228228275596796362751.
%C a(n) is not of the form 5*k+6 (divisibility by 11) or 9*k+8 (divisibility by 19) or 7*k+3*(1)^k (divisibility by 29).
%C Conjecture: the odd terms are not of the form 8*k+7.
%C n is in the sequence iff 2*n is in A098845 (terms a(21)a(30) are derived from that sequence).  _Ray Chandler_, Sep 25 2019
%e 5 is in the sequence because 16^54^51 = 1047551 is prime.
%t Select[Range[2500], PrimeQ[16^#  4^#  1] &]
%o (Magma) [n: n in [0..500]  IsPrime(16^n4^n1)];
%o (PARI) is(n)=ispseudoprime(16^n4^n1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A098845, similar sequences listed in A265481.
%K nonn,more
%O 1,2
%A _Vincenzo Librandi_, Dec 10 2015
