Let A and B be any sets with empty intersection, and let left bracketing bar X right bracketing bar denote the cardinal number of a set X. Then left bracketing bar A right bracketing bar + left bracketing bar B right bracketing bar = left bracketing bar A union B right bracketing bar (Ciesielski 1997, p. 68; Dauben 1990, p. 173; Rubin 1967, p. 274; Suppes 1972, pp. 112-113).

cardinal exponentiation | cardinal multiplication