a(n) = (-7*3^n+(-3)^n+6*4^n) / 42.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =25a(5) =100a(6) =481a(7) =1924a(8) =8425a(9) =33700a(10) =141361a(11) =565444a(12) =2320825a(13) =9283300a(14) =37664641a(15) =150658564a(16) =607417225a(17) =2429668900a(18) =9761722321a(19) =39046889284a(20) =156574977625a(21) =626299910500