a(n) = 1 + n + binomial(n+3,5).

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =26a(5) =62a(6) =133a(7) =260a(8) =471a(9) =802a(10) =1298a(11) =2014a(12) =3016a(13) =4382a(14) =6203a(15) =8584a(16) =11645a(17) =15522a(18) =20368a(19) =26354a(20) =33670a(21) =42526a(22) =53153a(23) =65804a(24) =80755a(25) =98306a(26) =118782a(27) =142534a(28) =169940a(29) =201406