a(n) = 11*3^n - 2*n - 10.

Terms

a(0) =1a(1) =21a(2) =85a(3) =281a(4) =873a(5) =2653a(6) =7997a(7) =24033a(8) =72145a(9) =216485a(10) =649509a(11) =1948585a(12) =5845817a(13) =17537517a(14) =52612621a(15) =157837937a(16) =473513889a(17) =1420541749a(18) =4261625333a(19) =12784876089a(20) =38354628361a(21) =115063885181a(22) =345191655645