a(n) = 4^n + 7^n + 9^n.

Terms

a(0) =3a(1) =20a(2) =146a(3) =1136a(4) =9218a(5) =76880a(6) =653186a(7) =5622896a(8) =48877058a(9) =428036240a(10) =3770308226a(11) =33362580656a(12) =296287600898