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

Terms

a(0) =2a(1) =7a(2) =40a(3) =199a(4) =856a(5) =3359a(6) =12440a(7) =44335a(8) =153808a(9) =523159a(10) =1752928a(11) =5804759a(12) =19041608a(13) =61981807a(14) =200458504a(15) =644783071a(16) =2064276256a(17) =6581953703a(18) =20911793168a(19) =66230028871a(20) =209167217752a(21) =658918365247