About this sequence (it will be A123015): 1, 2, 3, 4, 6, 8, 10, 13, 17, 21, 26, 33, 41, 50, 62, 77, 94, 115, 142, 174, 212, 260, 319, 389, 475, 582, 711, 867, 1060, 1296 Its a binary tree where leafs is getting added when a nodes internal counter reaches 3. - I really don't understand the definition. What exactly is the n-th term? Neil Sloane