51 (number)
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51 is a natural number that comes after 50 and before 52. It is a composite number, meaning it has factors other than one and itself. Its prime factorization is 3 × 17, and it is therefore classified as a semiprime — a number produced by the multiplication of two prime numbers.
Its divisors are 1, 3, 17, and 51. The sum of all divisors is 72. Excluding itself, the sum of its divisors (1 + 3 + 17) is 21, which is less than 51, making 51 a deficient number in the classical scheme of number classification.
Mathematical properties
51 is a pentagonal number: the formula n(3n − 1)/2 with n = 6 yields 51. It is also not divisible by 2 and is an odd number. The count of prime numbers less than 51 is 15. Euler's totient function, φ(51), is 32, representing the quantity of integers from 1 to 51 that share no common factor with 51.
Representation in different number systems
In Roman numerals, 51 is written as LI. In binary it is 110011, which reads the same in both directions (a palindrome). In hexadecimal it is 33, while in octal it is 63.
Other uses
The number 51 appears in various numbering systems: historical dates (such as 51 CE), road numbers, sports team numbers, and place designators. A well-known example is Area 51, a designation for a U.S. military facility in Nevada.