a(n) = C(n,5) + C(n,4) - C(n,3) + 1, n >= 7.

Terms

a(0) =3a(1) =22a(2) =71a(3) =169a(4) =343a(5) =628a(6) =1068a(7) =1717a(8) =2640a(9) =3914a(10) =5629a(11) =7889a(12) =10813a(13) =14536a(14) =19210a(15) =25005a(16) =32110a(17) =40734a(18) =51107a(19) =63481a(20) =78131a(21) =95356a(22) =115480a(23) =138853a(24) =165852a(25) =196882a(26) =232377a(27) =272801a(28) =318649a(29) =370448