a(n) = 4*a(n-2) - a(n-4) for n > 1, a(n) = n for n = 0, 1.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =4a(5) =11a(6) =15a(7) =41a(8) =56a(9) =153a(10) =209a(11) =571a(12) =780a(13) =2131a(14) =2911a(15) =7953a(16) =10864a(17) =29681a(18) =40545a(19) =110771a(20) =151316a(21) =413403a(22) =564719a(23) =1542841a(24) =2107560a(25) =5757961a(26) =7865521a(27) =21489003a(28) =29354524a(29) =80198051