835

835 = 5 x 167.

835 is the ninth Motzkin number.

835 is 343 in base 16.

835 is the sum of six positive cubes in exactly three ways.

Inn at 835 is a historic hotel in Springfield, Illinois.

Source: On-Line Encyclopedia of Integer Sequences

21

21 = 3 x 7.

21 is 10101 in base 2 (binary) and 111 in base 4.

21 has a unique representation as a sum of three squares: 21 = 1² + 2² + 4².

21 is the smallest number with five representations as a sum of three primes: 21 = 2 + 2 + 17 = 3 + 5 + 13 = 3 + 7 + 11 = 5 + 5 + 11 = 7 + 7 + 7.

21 is the eighth Fibonacci number.

21 is the fifth Motzkin number.

21 is the sixth triangular number.

21 is the total number of spots on a normal die.

Source: Phillips, R. 2004. Numbers: Facts, Figures and Fiction. Badsey Publications.

51

51 = 3 x 17.

51 is the smallest number that can be written with all the digits from 1 to 5 (without repetition) as a sum of prime numbers: 2 + 3 + 5 + 41 = 51. Note that the highest digit (5) and the lowest digit (1) are the digits of 51.

51 is a pentagonal number, a number of the form n(3n - 1)/2, and a centered pentagonal number (one of the few numbers to be both).

51 is 110011 in base 2 (binary) and 33 in base 16.

51 is the sixth Motzkin number.

Area 51 is a nickname for a secret military base located in the southern portion of Nevada in the western United States.

Source: Prime Curios!