@article{10_4153_CMB_1969_095_x,
     author = {Gr\"atzer, G. and Lakser, H. and P{\l}onka, J.},
     title = {Joins and {Direct} {Products} of {Equational} {Classes}},
     journal = {Canadian mathematical bulletin},
     pages = {741--744},
     year = {1969},
     volume = {12},
     number = {6},
     doi = {10.4153/CMB-1969-095-x},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1969-095-x/}
}