a(n) = floor(1/{(n^4+3*n)^(1/4)}), where {}=fractional part.

Terms

a(0) =2a(1) =6a(2) =12a(3) =21a(4) =33a(5) =48a(6) =65a(7) =85a(8) =108a(9) =133a(10) =161a(11) =192a(12) =225a(13) =261a(14) =300a(15) =341a(16) =385a(17) =432a(18) =481a(19) =533a(20) =588a(21) =645a(22) =705a(23) =768a(24) =833a(25) =901a(26) =972a(27) =1045a(28) =1121a(29) =1200