817

817 = 19 x 43.

817 is a centered hexagonal (hex) number (A003215).

817 is the sum of three consecutive prime numbers: 817 = 269 + 271 + 277.

817 has the representation 817 = 3⁶ + 88.

The sum of the even digits of 817 equals the sum of the odd digits of 817 (A036301).

817 is a number n such that 1 + n² + n⁴ + n⁶ + n⁷ is a prime (A126908).

Source: On-Line Encyclopedia of Integer Sequences

721

721 = 7 x 103.

721 is a centered hexagonal number (A003215).

721 is the smallest number that can be written as the difference of two positive cubes in two ways (A014440).

721 is the sum of nine consecutive primes: 721 = 61 + 67 + 71 + 73 + 79 + 83 + 89 + 97 + 101.

721 is the sum of the factorials of the digits of 16 (A061602): 721 = 6! + 1!

Source: What's Special About This Number?

547

547 is a prime number.

547 is a centered hexagonal number (or hex number) and a centered heptagonal number.

The number of primes between 6! and 7! is itself prime. 547 is the smallest odd prime to enumerate the primes between n! and (n + 1)! for any n.

547 is the smallest number that cannot be written using 11 copies of 11 and the operations +, -, x, /.

547 is 202021 in base 3 and 20203 in base 4. It is 4142 in base 4.

Source: Prime Curios!