a(n) = 4*3^n/3 - 5*0^n/6 - (n-1)2^(n-1).

Terms

a(0) =1a(1) =4a(2) =10a(3) =28a(4) =84a(5) =260a(6) =812a(7) =2532a(8) =7852a(9) =24196a(10) =74124a(11) =225956a(12) =686060a(13) =2076612a(14) =6270796a(15) =18902500a(16) =56904108a(17) =171138308a(18) =514332428a(19) =1544963364a(20) =4639084396a(21) =13926166084a(22) =41797372620a(23) =125431963748a(24) =376379777324