@article{10_21136_CMJ_1970_100972,
     author = {Havel, V\'aclav},
     title = {Free extensions of $(a,b)$-systems},
     journal = {Czechoslovak Mathematical Journal},
     pages = {353--356},
     publisher = {mathdoc},
     volume = {20},
     number = {3},
     year = {1970},
     doi = {10.21136/CMJ.1970.100972},
     mrnumber = {0265264},
     zbl = {0216.43003},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CMJ.1970.100972/}
}