a(n) = ((6 + sqrt(2))^n + (6 - sqrt(2))^n)/2.

Terms

a(0) =1a(1) =6a(2) =38a(3) =252a(4) =1732a(5) =12216a(6) =87704a(7) =637104a(8) =4663312a(9) =34298208a(10) =253025888a(11) =1870171584a(12) =13839178816a(13) =102484311936a(14) =759279663488