523

Number
$523$ (five hundred and twenty-three) is:


 * The $99$th prime number


 * The $2$nd of the $25$th pair of twin primes:
 * $523 - 521 = 2$


 * The $1$st element of the smallest sequence of $3$ consecutive positive integers whose Euler $\phi$ values are strictly decreasing:
 * $\phi \left({523}\right) = 522$, $\phi \left({524}\right) = 260$, $\phi \left({525}\right) = 240$

Also see

 * First Sequence of Three Consecutive Strictly Decreasing Euler Phi Values