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!

4665

4665 = 3 x 5 x 311.

4665 is 33333 in base 6.

4665 is a divisor of 52²⁰ - 1.

The sum of the squares of the digits of 4665 is equal to the largest prime factor reversed, where the largest prime factor is not a palindrome: 4² + 6² + 6² + 5² = 113.

IC 4665 is one of the brighter clusters in the Milky Way galaxy.

Source: What's Special About This Number?

460

460 = 2² x 5 x 23.

460 has a unique representation as a sum of three squares: 460 = 6² + 10² + 18².

460 is a centered triangular number.

460 is the sum of twelve consecutive primes: 460 = 17 + 19 + 23 + 29 + 31 + 37 + 41 + 43 + 47 + 53 + 59 + 61.

460 Scania is an asteroid discovered in 1900.

Source: Wikipedia

3546

3546 = 2 x 3² x 197.

3546 has a representation as a sum of two squares: 3546 = 39² + 45².

3546 is a divisor of 19¹⁴ - 1.

3546 is 1414 in base 14.

3546 is a number such that it together with its double and triple contain all digits: 3546, 7092, and 10638.

3546 is the maximum length of a rook tour on a 17 x 18 board.

Source: On-Line Encyclopedia of Integer Sequences

354

354 = 2 x 3 x 59.

354 is the sum of the first four fourth powers: 354 = 1⁴ + 2⁴ + 3⁴ + 4⁴.

354 is the smallest number whose first two digits are distinct odd primes and whose third digit is the arithmetic mean of the first two.

354 is the sum of two triangular numbers in exactly two ways: 354 = 351 + 3 = 276 + 78.

354 is the average number of days in a lunar year (rounded off).

Source: Number Gossip

947

947 is a prime number. The difference between any two of its digits is also a prime.

947 is a Chen prime because 947 + 2 = 949 = 13 x 73, so 949 is a semiprime.

947 is the maximum number of regions into which 43 lines divide the plane.

947 is the sum of seven consecutive primes: 947 = 113 + 127 + 131 + 137 + 139 + 149 + 151.

947 is the telephone area code for Oakland, Michigan. Note that the words "telephone code Oakland" have, respectively, 9, 4, and 7 digits.

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?