Diagonal length function.

Terms

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