a(n) = 40*n^2 - 1.

Terms

a(0) =39a(1) =159a(2) =359a(3) =639a(4) =999a(5) =1439a(6) =1959a(7) =2559a(8) =3239a(9) =3999a(10) =4839a(11) =5759a(12) =6759a(13) =7839a(14) =8999a(15) =10239a(16) =11559a(17) =12959a(18) =14439a(19) =15999a(20) =17639a(21) =19359a(22) =21159a(23) =23039a(24) =24999a(25) =27039a(26) =29159a(27) =31359a(28) =33639a(29) =35999