a(n) = 5^n - n^2.

Terms

a(0) =1a(1) =4a(2) =21a(3) =116a(4) =609a(5) =3100a(6) =15589a(7) =78076a(8) =390561a(9) =1953044a(10) =9765525a(11) =48828004a(12) =244140481a(13) =1220702956a(14) =6103515429a(15) =30517577900a(16) =152587890369a(17) =762939452836