a(n) = 6^n - n^4.

Terms

a(0) =1a(1) =5a(2) =20a(3) =135a(4) =1040a(5) =7151a(6) =45360a(7) =277535a(8) =1675520a(9) =10071135a(10) =60456176a(11) =362782415a(12) =2176761600a(13) =13060665455a(14) =78364125680a(15) =470184933951