523 is a prime number.
521 and 523 form a twin prime pair.
523 together with 541 form the smallest two consecutive primes such that the sums of their digits are equal. It is the smallest prime that is followed immediately by 17 composite numbers.
523 is the only three-digit prime containing all three of the first three prime digits. It is the largest prime with a prime number of distinct prime digits. It is the only prime formed from three consecutive primes, one being the sum of the other two.
523 is 313 in base 13.
523 is the sum of seven consecutive primes: 523 = 61 + 67 + 71 + 73 + 79 + 83 + 89.
Source: Number Gossip