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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =5a(5) =11a(6) =25a(7) =56a(8) =126a(9) =283a(10) =636a(11) =1429a(12) =3211a(13) =7215a(14) =16212a(15) =36428a(16) =81853a(17) =183922a(18) =413269a(19) =928607a(20) =2086561a(21) =4688460a(22) =10534874a(23) =23671647a(24) =53189708a(25) =119516189a(26) =268550439a(27) =603427359a(28) =1355888968a(29) =3046654856