Hello Math-Fun ... 1199, 1200, 1220, 1222, 1224, 1248,... If we compute a(n) - a(n+1) we notice that the result is a substring of both a(n) and a(n+1). The above example gives: 1199, 1200, 1220, 1222, 1224, 1248,... 1 20 2 2 24 Question: is an infinite such seq possible? If yes, what would a(1) be of the lexico- first one (of distinct terms)? Note that it is always possible to build a seq W as long as wanted: just start W with 1 followed by a zillion zeroes -- and add 1 at each step. This might be old hat, sorry, but I don't know how to search the OEIS for seqs where I cannot compute at least a(1)... Best, É.