Inaccessible cardinal
CARDINAL UNOBTAINIBLE FROM SMALLER CARDINALS VIA USUAL CARDINAL ARITHMETIC
Strongly inaccessible cardinal; Weakly inaccessible cardinal; Inaccessible cardinals axiom; Hyper-inaccessible cardinal; Inaccessible cardinals; Accessible cardinal; Weakly inaccessible; Strongly inaccessible
In set theory, an uncountable cardinal is inaccessible if it cannot be obtained from smaller cardinals by the usual operations of cardinal arithmetic. More precisely, a cardinal is strongly inaccessible if it is uncountable, it is not a sum of fewer than cardinals smaller than , and \alpha < \kappa implies 2^{\alpha} < \kappa.