a(n) = (11*4^n + 1)/3.

Terms

a(0) =4a(1) =15a(2) =59a(3) =235a(4) =939a(5) =3755a(6) =15019a(7) =60075a(8) =240299a(9) =961195a(10) =3844779a(11) =15379115a(12) =61516459a(13) =246065835a(14) =984263339a(15) =3937053355a(16) =15748213419a(17) =62992853675a(18) =251971414699