a(n) = 2*a(n-1) - a(n-3) + 2*a(n-4).

Terms

a(0) =0a(1) =1a(2) =3a(3) =8a(4) =15a(5) =29a(6) =56a(7) =113a(8) =227a(9) =456a(10) =911a(11) =1821a(12) =3640a(13) =7281a(14) =14563a(15) =29128a(16) =58255a(17) =116509a(18) =233016a(19) =466033a(20) =932067a(21) =1864136a(22) =3728271a(23) =7456541a(24) =14913080a(25) =29826161a(26) =59652323a(27) =119304648a(28) =238609295a(29) =477218589