a(n) = (14*4^n + 1)/3.

Terms

a(0) =5a(1) =19a(2) =75a(3) =299a(4) =1195a(5) =4779a(6) =19115a(7) =76459a(8) =305835a(9) =1223339a(10) =4893355a(11) =19573419a(12) =78293675a(13) =313174699a(14) =1252698795a(15) =5010795179a(16) =20043180715a(17) =80172722859a(18) =320690891435