A Kolakoski sequence, named after William Kolakoski, is an infinite sequence of digits whose run lengths reduce to the sequence itself.
For example, the Kolakoski sequence for the numbers 1 and 2 is 1,2,2,1,1,2,1,2,2,1,2,2,1,1,2,1,1,2,2,1,2,1,1,2,1,2,2,1,1… because, when writing down the successive run lengths of 1s and 2s, you get the same sequence back: