OFFSET
1,3
COMMENTS
Zero together with orders of finite abelian groups that appear as the group of units in a commutative ring (Chebolu and Lockridge).
Also the possible number of units in a (commutative or non-commutative) ring, since every odd number that is the number of units of a ring must be in this sequence (Ditor's theorem, stated in the S. Chebolu and K. Lockridge link). - Jianing Song, Dec 24 2021
LINKS
S. Chebolu and K. Lockridge, How Many Units Can a Commutative Ring Have?, Amer. Math. Monthly, 124 (2017), 960-965; arXiv, arXiv:1701.02341 [math.AC], 2017.
EXAMPLE
The even integers {0, +-2, +-4, ...} form a commutative ring with no (multiplicative) units, so a(1) = 0.
CROSSREFS
KEYWORD
nonn
AUTHOR
Jonathan Sondow, Dec 14 2017
STATUS
approved