We also present a history of selected later developments, including work of Mike
Fredman, Don Knuth, David Klarner, and Don Coppersmith. Their work addresses
the structure of particular affine integer semigroups, and sufficient conditions for an
integer affine semigroup to be freely generated. The latter topic led Klarner to pose in
1982 several easy-to-state problems in the spirit of Erd˝os’s prize problem, given at the
end of the paper, which remain unsolved.