a(n) = least positive integer > a(n-1) and not a(i)^2 + a(j)^2 + a(k)^2 for 1<=i<=j<=k<=n.

Terms

a(0) =1a(1) =2a(2) =4a(3) =5a(4) =7a(5) =8a(6) =10a(7) =11a(8) =13a(9) =14a(10) =15a(11) =16a(12) =17a(13) =19a(14) =20a(15) =22a(16) =23a(17) =25a(18) =26a(19) =28a(20) =29a(21) =31a(22) =32a(23) =34a(24) =35a(25) =37a(26) =38a(27) =39a(28) =40a(29) =41