a(n) = 2*a(n-1) + 2*a(n-2) + 2*a(n-3) - a(n-4) with a(0)=0, a(1)=1, a(2)=2, a(3)=7.

Terms

a(0) =0a(1) =1a(2) =2a(3) =7a(4) =20a(5) =57a(6) =166a(7) =479a(8) =1384a(9) =4001a(10) =11562a(11) =33415a(12) =96572a(13) =279097a(14) =806606a(15) =2331135a(16) =6737104a(17) =19470593a(18) =56271058a(19) =162626375a(20) =469998948a(21) =1358322169a(22) =3925623926a(23) =11345263711a(24) =32788420664a(25) =94760294433a(26) =273862333690a(27) =791476833863