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

Terms

a(0) =2a(1) =7a(2) =29a(3) =133a(4) =641a(5) =3157a(6) =15689a(7) =78253a(8) =390881a(9) =1953637a(10) =9766649a(11) =48830173a(12) =244144721a(13) =1220711317a(14) =6103532009a(15) =30517610893a(16) =152587956161a(17) =762939584197