a(n) = A027113(n, n+3).

Terms

a(0) =7a(1) =23a(2) =56a(3) =120a(4) =241a(5) =467a(6) =886a(7) =1660a(8) =3087a(9) =5715a(10) =10552a(11) =19452a(12) =35825a(13) =65943a(14) =121342a(15) =223240a(16) =410663a(17) =755391a(18) =1389448a(19) =2555664a(20) =4700673a(21) =8645963a(22) =15902486a(23) =29249316a(24) =53797967a(25) =98949979a(26) =181997480a(27) =334745652a(28) =615693345