a(n) = 4*a(n - 1) + 2*a(n - 2) for n>1, a(0)=3, a(1)=5.

Terms

a(0) =3a(1) =5a(2) =26a(3) =114a(4) =508a(5) =2260a(6) =10056a(7) =44744a(8) =199088a(9) =885840a(10) =3941536a(11) =17537824a(12) =78034368a(13) =347213120a(14) =1544921216a(15) =6874111104a(16) =30586286848a(17) =136093369600a(18) =605546052096