Hi everyone, a(n) is the least positive integer not occurring earlier in the sequence such that, if a(m) = a(n)+1, then |m - n| = a(n). We start with a(1) = 1, then a(2) = 2. We move ahead and get a(4) = 3, and a(7) = 4. Now we go back to a(3) = 5 and that forces a(8) to be 6, etc. Best, Ali