551

551 = 19 x 29.

551 is the sum of three consecutive primes: 551 = 179 + 181 + 191.

551 is a centered decagonal number.

551 is the number of trees with 12 vertices.

551 is 202102 in base 3 and 20213 in base 4.

551 is a divisor of 30³ - 1.

551 Ortrud is an asteroid discovered in 1904.

Source: What's Special About This Number?

781

781 = 11 x 71.

781 is a divisor of 5⁵ - 1.

781 is 11111 in base 5.

781 is a centered decagonal number.

781 is the maximum number of regions into which 39 lines divide the plane.

The oldest recorded eruption of Mt. Fuji in Japan occurred in the year 781.

Source: Positive Integers

211

211 is a prime number.

211 is the smallest prime such that the sum of its even digits equals the sum of its odd digits.

211 is 111 in base 14.

211 is a centered decagonal number.

211 is the maximum number of regions into which 20 lines divide the plane.

211 is the number of primes that can appear on a 24-hour digital clock starting from 00:00 up to 23:59.

Source: Number Gossip

661

661 is a prime number.

661 has a representation as a sum of two squares: 661 = 6² + 25².

659 and 661 form a twin prime pair.

661 is the hypotenuse of a primitive Pythagorean triple: 661² = 300² + 589².

661 is a centered decagonal number.

Physicist William Thomson, Lord Kelvin (1824-1907) published 661 papers on a wide range of scientific topics.

Source: Prime Curios!

451

451 = 11 x41.

451 is a decagonal number. It is also a centered decagonal number.

10⁴⁵¹ + 451 is a prime number.

3 plus the integer formed by the first 451 digits after the decimal point in pi is a prime number.

The title of the novel and film Fahrenheit 451 refers to the temperature at which book paper supposedly burns.

Source: Prime Curios!