要长话短说的话, 请去:
"Unending Chess, Symbolic Dynamics and a Problem in Semigroups"
Duke mathematics Journal 11 (1944):1-7
by Marston Morse and Gustav Hedlund
看.
或者:
"A Problem on Strings of Beads"
by John Leech
Mathematical Gazette 41 (1957):277-278
//grin