a(n) = 3*a(n-1) + C(n+3,3) for n > 0; a(0)=1.

Terms

a(0) =1a(1) =7a(2) =31a(3) =113a(4) =374a(5) =1178a(6) =3618a(7) =10974a(8) =33087a(9) =99481a(10) =298729a(11) =896551a(12) =2690108a(13) =8070884a(14) =24213332a(15) =72640812a(16) =217923405a(17) =653771355a(18) =1961315395a(19) =5883947725a(20) =17651844946a(21) =52955536862a(22) =158866612886a(23) =476599841258