@article{oai:tsukuba.repo.nii.ac.jp:00016243,
 author = {Abe, Eiichi},
 journal = {Tsukuba Journal of Mathematics},
 month = {Dec},
 pages = {7--26},
 title = {A generalization of groups with a root data and coverings of the groups},
 volume = {1},
 year = {1977}
}