If n-th triangular number (A000217(n)) is odd, multiply it by 4; if even, multiply it by 5.

Terms

a(0) =4a(1) =12a(2) =30a(3) =50a(4) =60a(5) =84a(6) =140a(7) =180a(8) =180a(9) =220a(10) =330a(11) =390a(12) =364a(13) =420a(14) =600a(15) =680a(16) =612a(17) =684a(18) =950a(19) =1050a(20) =924a(21) =1012a(22) =1380a(23) =1500a(24) =1300a(25) =1404a(26) =1890a(27) =2030a(28) =1740a(29) =1860