a(n) = 5*2^n + 3^n + 15.

Terms

a(0) =21a(1) =28a(2) =44a(3) =82a(4) =176a(5) =418a(6) =1064a(7) =2842a(8) =7856a(9) =22258a(10) =64184a(11) =187402a(12) =551936a(13) =1635298a(14) =4864904a(15) =14512762a(16) =43374416a(17) =129795538a(18) =388731224a(19) =1164882922a(20) =3492027296a(21) =10470838978a(22) =31402031144a(23) =94185121882a(24) =282513422576a(25) =847456381618