a(n) = 3*a(n-1) - 4*a(n-2) + 6*a(n-3) - 4*a(n-4), with initial terms 0,0,0,1.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =5a(6) =9a(7) =21a(8) =45a(9) =85a(10) =165a(11) =341a(12) =693a(13) =1365a(14) =2709a(15) =5461a(16) =10965a(17) =21845a(18) =43605a(19) =87381a(20) =174933a(21) =349525a(22) =698709a(23) =1398101a(24) =2796885a(25) =5592405a(26) =11183445a(27) =22369621a(28) =44741973a(29) =89478485