a(n) = 2*a(n-1) + a(n-2) - a(n-4) - a(n-5) - a(n-6) - a(n-7).

Terms

a(0) =2a(1) =5a(2) =12a(3) =28a(4) =65a(5) =150a(6) =346a(7) =795a(8) =1826a(9) =4192a(10) =9621a(11) =22078a(12) =50660a(13) =116239a(14) =266704a(15) =611930a(16) =1404013a(17) =3221358a(18) =7391048a(19) =16957921a(20) =38908004a(21) =89269924a(22) =204819503a(23) =469934590a(24) =1078210352a(25) =2473828397a(26) =5675911794a(27) =13022719964