F. M. Hall: An Introduction into Abstract Algebra. 2nd edition, CUP ARchive 1980, ISBN 978-0-521-29861-2, p. 101 (online copy,第101頁,載於Google圖書)
Bonnie Averbach, Orin Chein: Problem Solving Through Recreational Mathematics. Courier Dover Publications 2000, ISBN 0-486-40917-1, pp. 125-127 (online copy,第125頁,載於Google圖書)
Talal Ghannam: The Mystery of Numbers: Revealed Through Their Digital Root. CreateSpace Publications 2012, ISBN 978-1477678411, pp. 68-73
T. H. O'Beirne: Puzzles and Paradoxes. In: New Scientist, No. 230, 1961-4-13, pp. 53-54 (online copy,第53頁,載於Google圖書)¨
外部連結
pattern of digital root using MS Excel (页面存档备份,存于互联网档案馆)