2*3^n-(-1)^n.

Terms

a(0) =1a(1) =7a(2) =17a(3) =55a(4) =161a(5) =487a(6) =1457a(7) =4375a(8) =13121a(9) =39367a(10) =118097a(11) =354295a(12) =1062881a(13) =3188647a(14) =9565937a(15) =28697815a(16) =86093441a(17) =258280327a(18) =774840977a(19) =2324522935a(20) =6973568801a(21) =20920706407a(22) =62762119217a(23) =188286357655a(24) =564859072961