a(n) = 15*binomial(n,6)-6*binomial(n-2,4)+binomial(n-4,4).

Terms

a(0) =0a(1) =0a(2) =9a(3) =75a(4) =331a(5) =1055a(6) =2745a(7) =6209a(8) =12670a(9) =23886a(10) =42285a(11) =71115a(12) =114609a(13) =178165a(14) =268541a(15) =394065a(16) =564860a(17) =793084a(18) =1093185a(19) =1482171a(20) =1979895a(21) =2609355a(22) =3397009a(23) =4373105a(24) =5572026a(25) =7032650a(26) =8798725a(27) =10919259a(28) =13448925a(29) =16448481