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

Terms

a(0) =1a(1) =1a(2) =4a(3) =7a(4) =18a(5) =38a(6) =88a(7) =195a(8) =441a(9) =988a(10) =2223a(11) =4992a(12) =11220a(13) =25208a(14) =56645a(15) =127277a(16) =285992a(17) =642615a(18) =1443946a(19) =3244514a(20) =7290360a(21) =16381287a(22) =36808421a(23) =82707768a(24) =185842671a(25) =417584688a(26) =938304280a(27) =2108350576a(28) =4737420745a(29) =10644887785