a(1) = 3; for n > 1, a(n) = 4*a(n-1) + 4 - n.

Terms

a(0) =3a(1) =14a(2) =57a(3) =228a(4) =911a(5) =3642a(6) =14565a(7) =58256a(8) =233019a(9) =932070a(10) =3728273a(11) =14913084a(12) =59652327a(13) =238609298a(14) =954437181a(15) =3817748712a(16) =15270994835a(17) =61083979326a(18) =244335917289a(19) =977343669140