a(n) = 10*(4^n - 1)/3 + 1.

Terms

a(0) =1a(1) =11a(2) =51a(3) =211a(4) =851a(5) =3411a(6) =13651a(7) =54611a(8) =218451a(9) =873811a(10) =3495251a(11) =13981011a(12) =55924051a(13) =223696211a(14) =894784851a(15) =3579139411a(16) =14316557651a(17) =57266230611a(18) =229064922451a(19) =916259689811