671

671 = 11 x 61.

671 is a rhombic dodecahedral number.

671 is the 3-digit composite number (k) that yields the greatest percentage of primes from Leonhard Euler's formula x² + x + k (453 out of 1000 possible primes).

671!/176! + 1 is prime.

671 is the magic constant for an 11 x 11 normal magic square.

671 is the solution for the n-queens problem, for n = 11.

671 is a divisor of 9⁵ - 1.

Source: Prime Curios!

553

553 = 7 x 79.

553 is a rhombic dodecahedral number.

553 is the sum of nine consecutive primes: 553 = 43 + 47 + 53 + 59 + 61 + 67 + 71 + 73 + 79.

Inserting a zero between adjacent digits of 553 produces a prime: 50503.

The CN Tower in Toronto is about 553 meters tall.

Source: What's Special About This Number?