k appears k times (k even).

Terms

a(0) =2a(1) =2a(2) =4a(3) =4a(4) =4a(5) =4a(6) =6a(7) =6a(8) =6a(9) =6a(10) =6a(11) =6a(12) =8a(13) =8a(14) =8a(15) =8a(16) =8a(17) =8a(18) =8a(19) =8a(20) =10a(21) =10a(22) =10a(23) =10a(24) =10a(25) =10a(26) =10a(27) =10a(28) =10a(29) =10