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

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =2a(6) =1a(7) =2a(8) =8a(9) =17a(10) =22a(11) =22a(12) =33a(13) =78a(14) =156a(15) =233a(16) =298a(17) =442a(18) =833a(19) =1546a(20) =2464a(21) =3553a(22) =5390a(23) =9230a(24) =16161a(25) =26358a(26) =40404a(27) =62713a(28) =103298a(29) =174290