@article{10_1017_S0017089500008879,
     author = {Humphries, Stephen P.},
     title = {Normal closures of powers of {Dehn} twists in mapping class groups},
     journal = {Glasgow mathematical journal},
     pages = {314--317},
     year = {1992},
     volume = {34},
     number = {3},
     doi = {10.1017/S0017089500008879},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500008879/}
}