Another infinite cardinal number is the number of numbers in the set of real numbers, and is represented by the symbol formula_4 (or formula_5, the cardinality of the continuum). Cantor proved that there are many different infinite cardinal numbers that are bigger than formula_1. A famous theorem of Cantor is that the cardinality of the real numbers is larger than the cardinality of the natural numbers. The continuum hypothesis is the statement that there is no middle cardinal number strictly between that of natural numbers and real numbers.