a(n) = floor(T(n+1)!*T(n-1)!/(T(n)!)^2), where T(n) = n(n+1)/2 = the n-th triangular number.

Terms

a(0) =6a(1) =20a(2) =42a(3) =71a(4) =108a(5) =152a(6) =204a(7) =263a(8) =330a(9) =403a(10) =485a(11) =573a(12) =669a(13) =773a(14) =884a(15) =1002a(16) =1128a(17) =1261a(18) =1401a(19) =1549a(20) =1704a(21) =1866a(22) =2036a(23) =2214a(24) =2398a(25) =2591a(26) =2790a(27) =2997a(28) =3211a(29) =3433