a(n) = 2^n + 4^n + 6^n.

Terms

a(0) =3a(1) =12a(2) =56a(3) =288a(4) =1568a(5) =8832a(6) =50816a(7) =296448a(8) =1745408a(9) =10340352a(10) =61515776a(11) =366993408a(12) =2193563648a(13) =13127811072a(14) =78632615936a(15) =471258759168