a(n) = position of n^2 + (n+1)^2 + (n+2)^2 in A004432.

Terms

a(0) =1a(1) =4a(2) =13a(3) =26a(4) =44a(5) =69a(6) =98a(7) =137a(8) =179a(9) =227a(10) =278a(11) =338a(12) =402a(13) =470a(14) =541a(15) =622a(16) =707a(17) =796a(18) =891a(19) =991a(20) =1097a(21) =1206a(22) =1320a(23) =1443a(24) =1569a(25) =1697a(26) =1834a(27) =1974a(28) =2120a(29) =2269