Lajoie, Caroline; Mura, Roberta (November 2000), «What's in a name? A learning difficulty in connection with cyclic groups», For the Learning of Mathematics20 (3): 29–33, JSTOR40248334.
ncatlab.org
The infinite cyclic group written multiplicatively may be written C (most common), C∞ (the subscript being interpreted as the order of the group), or even C0 (to fit the statement Cn ≅ Z/nZ, n ≥ 0, as given here).