Perfect powers n such that (n-5)/2 is prime.

Terms

a(0) =9a(1) =27a(2) =2187a(3) =19683a(4) =300763a(5) =1092727a(6) =2048383a(7) =6128487a(8) =15069223a(9) =21717639a(10) =24642171a(11) =27818127a(12) =36264691a(13) =38958219a(14) =54439939a(15) =65450827a(16) =84604519a(17) =90224199a(18) =112678587a(19) =133432831a(20) =139798359a(21) =143055667a(22) =163667323a(23) =246491883a(24) =313046839a(25) =341532099a(26) =347428927