a(n) = 11*4^n + 1.

Terms

a(0) =12a(1) =45a(2) =177a(3) =705a(4) =2817a(5) =11265a(6) =45057a(7) =180225a(8) =720897a(9) =2883585a(10) =11534337a(11) =46137345a(12) =184549377a(13) =738197505a(14) =2952790017a(15) =11811160065a(16) =47244640257a(17) =188978561025a(18) =755914244097