@article{SF_1978__16_134296,
     author = {M.L. Howard},
     title = {Roots of doubly-infinite increasing sequences.},
     journal = {Semigroup forum},
     pages = {385--386},
     year = {1978},
     volume = {16},
     zbl = {0393.20053},
     url = {http://geodesic.mathdoc.fr/item/SF_1978__16_134296/}
}