Wondering whether anyone recognizes this binary tree: 0 1 2 3 6 9 12 4 5 7 8 10 11 13 14 15 30 45 ... 240 16 17 31 32 46 47 18 21 24 27 33 36 39 42 48 51 54 57 19 20 22 23 25 26 28 29 34 35 37 38 40 41 43 44 49 50 52 53 55 56 58 59 ... (To be clear, 15 and 30 are daughters of 4, 45 and 60 are daughters of 5, and so forth.) I would think the OEIS-appropriate version was 0,1,2,3,6,9,12,4,5,7,8... or 1,2,3,4,7,10,13,5,6,8,9... Those aren't in there, but I'm not sure that's the right way to encode the tree. The parent function (starting from child 1) is 0,0,1,3,3,1,6,6,2,9,9,2,12,12,4,15,15... What I'm really wondering is whether someone's done heapsort with this arrangement before. --Steve