388

388 = 2² x 97.

388 has a representation as a sum of two squares: 388 = 8² + 18².

The number 388 followed by 388 sevens is a prime number.

388 is the maximum value of n so that there exist 6 denominations of stamps so that every postage from 1 to n can be paid for with at most 6 stamps.

The Junkers Ju 388 was a German World War II aircraft.

Source: Prime Curios!

531

531 = 3² x 59.

531 is the smallest number with the property that its first four multiples contain the digit 1: 531, 1062, 1593, and 2124.

291,548,736 = 8 x 92 x 531 x 746. Both the number and its factors use each of the digits from 1 to 9 exactly once.

531 is 20103 in base 4 and 213 in base 16.

The construction of the St. Sophia Basilica in Constantinople began in the year 531.

Source: What's Special About This Number?

623

623 = 7 x 89.

623 is a number that cannot be written as a sum of three squares.

623 - 6*2*3 and 623 + 6*2*3 are prime numbers: 587 and 659.

623 is the sum of three distinct positive cubes: 623 = 4³ + 6³ + 7³.

A Uranus-Neptune conjunction in Virgo occurred in the year 623.

Source: Prime Curios!

205

205 = 5 x 41.

205 has two representations as a sum of two squares: 205 = 3² + 14² = 6² + 13².

205 is the hypotenuse of two primitive Pythagorean triples: 205² = 84² + 187² = 133² + 156².

Every number greater than 205 is the sum of distinct primes of the form 6n + 1.

205 is the number of pairs of twin primes less than 10,000.

The Peugeot 205 is a supermini produced by the French car manufacturer Peugeot between 1983 and 1997.

Source: Prime Curios!

3.14159...

3.14159... (π or pi) is the ratio of a circle's circumference to its diameter.

π is the area of a unit circle. It is also the ratio of the circumference of any convex curve of constant diameter to its diameter (Barbier's theorem).

π is an irrational number and a transcendental number.

22/7 is the best approximation to π, using the ratio of two numbers less than 100.

In 1949, the ENIAC calculated π to 2037 places in 70 hours, without making any mistakes. The current record is nearly 2.7 trillion decimal digits, confirmed in January 2010.

Source: Wikipedia

535

535 = 5 x 107

5, 3, and 5 are the eighth, ninth, and tenth decimal digits of pi.

535 is a number that cannot be written as a sum of three squares.

535⁵³⁵ + (535 - 1) is a prime number.

535 is the smallest palindrome such that its cube can be written as a sum of three prime cubes: 535³ = 349³ + 379³ + 383³.

The Indonesian volcano Krakatoa erupted in the year 535.

Source: Prime Curios!

53

53 is a prime number. It is also a Sophie Germain prime because 2 x 53 + 1 = 107, which is also a prime.

5 and 3 are the eighth and ninth decimal digits of pi.

53 has a representation as a sum of two squares: 53 = 2² + 7².

53 is the hypotenuse of a primitive Pythagorean triple: 53² = 28² + 45².

53 is the sum of five consecutive primes: 5 + 7 + 11 + 13 + 17 = 53.

The chemical element iodine has atomic number 53.

Source: Prime Curios!