a(n) = 7*a(n-1) -14*a(n-2) +7*a(n-3), with a(0)=1, a(1)=2, a(2)=7.

Terms

a(0) =1a(1) =2a(2) =7a(3) =28a(4) =112a(5) =441a(6) =1715a(7) =6615a(8) =25382a(9) =97069a(10) =370440a(11) =1411788a(12) =5375839a(13) =20458921a(14) =77833217a(15) =296038498a(16) =1125816895a(17) =4281011812a(18) =16277915640a(19) =61891962377a(20) =235320000363a(21) =894697938743