Generalized Fibonacci numbers.

Terms

a(0) =1a(1) =1a(2) =4a(3) =13a(4) =53a(5) =228a(6) =1037a(7) =4885a(8) =23640a(9) =116793a(10) =586633a(11) =2986616a(12) =15377097a(13) =79927913a(14) =418852716a(15) =2210503285a(16) =11738292397a(17) =62673984492a(18) =336260313765