a(n) = 7*a(n-1) - 14*a(n-2) + 7*a(n-3) with a(0)=0, a(1)=1, a(2)=4.

Terms

a(0) =0a(1) =1a(2) =4a(3) =14a(4) =49a(5) =175a(6) =637a(7) =2352a(8) =8771a(9) =32928a(10) =124166a(11) =469567a(12) =1779141a(13) =6749211a(14) =25623472a(15) =97329337a(16) =369821228a(17) =1405502182a(18) =5342323441a(19) =20307982135a(20) =77201862045a(21) =293497548512