a(n) = 3*a(n-1) + 6*a(n-2), n>2, a(0)=1, a(1)=1, a(2)=5.

Terms

a(0) =1a(1) =1a(2) =5a(3) =21a(4) =93a(5) =405a(6) =1773a(7) =7749a(8) =33885a(9) =148149a(10) =647757a(11) =2832165a(12) =12383037a(13) =54142101a(14) =236724525a(15) =1035026181a(16) =4525425693a(17) =19786434165a(18) =86511856653a(19) =378254174949