James Anderson, Tom Head. The lattice of semiretracts of a free monoid. Int. J. Comput. Math., 43(3-4):127-131, 1992. [doi]
@article{AndersonH92,
title = {The lattice of semiretracts of a free monoid},
author = {James Anderson and Tom Head},
year = {1992},
doi = {10.1080/00207169208804078},
url = {https://doi.org/10.1080/00207169208804078},
researchr = {https://researchr.org/publication/AndersonH92},
cites = {0},
citedby = {0},
journal = {Int. J. Comput. Math.},
volume = {43},
number = {3-4},
pages = {127-131},
}