@article{10_4153_CJM_2015_023_9,
     author = {Bret\'on, David J. Fern\'andez},
     title = {Strongly {Summable} {Ultrafilters,} {Union} {Ultrafilters,} and the {Trivial} {Sums} {Property}},
     journal = {Canadian journal of mathematics},
     pages = {44--66},
     year = {2016},
     volume = {68},
     number = {1},
     doi = {10.4153/CJM-2015-023-9},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-2015-023-9/}
}