@article{10_4153_CMB_1974_087_5,
     author = {Duncan, Richard},
     title = {On {2-Summing} {Operators}},
     journal = {Canadian mathematical bulletin},
     pages = {493--498},
     year = {1974},
     volume = {17},
     number = {4},
     doi = {10.4153/CMB-1974-087-5},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1974-087-5/}
}