• 51 can be written using four 4's:

51 has 4 divisors (see below), whose sum is σ = 72. Its totient is φ = 32.

The previous prime is 47. The next prime is 53. The reversal of 51 is 15.

Added to its reverse (15) it gives a triangular number (66 = T_{11}).

51 is nontrivially palindromic in base 2, base 4 and base 16.

51 is an esthetic number in base 6, base 9 and base 12, because in such bases its adjacent digits differ by 1.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 15 = 3 ⋅5.

It is a 5-Lehmer number, since φ(51) divides (51-1)^{5}.

It is the 6-th Motzkin number.

It is a cyclic number.

It is a trimorphic number since its cube, 132651, ends in 51.

It is not a de Polignac number, because 51 - 2^{2} = 47 is a prime.

It is a D-number.

51 is an undulating number in base 4.

It is the 14-th Perrin number.

51 is a lucky number.

51 is a nontrivial repdigit in base 16.

It is a plaindrome in base 6, base 9, base 11, base 13, base 14, base 15 and base 16.

It is a nialpdrome in base 8, base 10, base 12 and base 16.

It is a zygodrome in base 2 and base 16.

A polygon with 51 sides can be constructed with ruler and compass.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6 + ... + 11.

It is an arithmetic number, because the mean of its divisors is an integer number (18).

51 is the 6-th pentagonal number.

51 is the 5-th centered pentagonal number.

51 is a deficient number, since it is larger than the sum of its proper divisors (21).

51 is a wasteful number, since it uses less digits than its factorization.

51 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 20.

The product of its digits is 5, while the sum is 6.

The square root of 51 is about 7.1414284285. The cubic root of 51 is about 3.7084297693.

Adding to 51 its reverse (15), we get a palindrome (66).

Subtracting from 51 its reverse (15), we obtain a triangular number (36 = T_{8}).

The spelling of 51 in words is "fifty-one", and thus it is an aban number and an uban number.

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