@article{oai:u-ryukyu.repo.nii.ac.jp:02000878,
 author = {Maehara, Hiroshi},
 journal = {European Journal of Combinatorics},
 month = {},
 note = {The purpose of this note is to present an elementary proof of the fact that no more than twelve unit balls can simultaneously touch a unit ball in 3-space,a proof that is accessible for undergraduates., 論文},
 title = {The problem of thirteen spheres : A proof for undergraduates},
 year = {2006}
}