51681 has 16 divisors (see below), whose sum is σ = 82944. Its totient is φ = 27984.

The previous prime is 51679. The next prime is 51683. The reversal of 51681 is 18615.

51681 = T₁₁₈ + T₁₁₉ + ... + T₁₂₄.

51681 is digitally balanced in base 2 and base 4, because in such bases it contains all the possibile digits an equal number of times.

51681 is a nontrivial binomial coefficient, being equal to C(322, 2).

It is an interprime number because it is at equal distance from previous prime (51679) and next prime (51683).

It is not a de Polignac number, because 51681 - 2¹ = 51679 is a prime.

It is a super-2 number, since 2×51681² = 5341851522, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (21).

51681 is a lucky number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 51681.

It is not an unprimeable number, because it can be changed into a prime (51683) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 430 + ... + 536.

It is an arithmetic number, because the mean of its divisors is an integer number (5184).

2⁵¹⁶⁸¹ is an apocalyptic number.

51681 is the 321-st triangular number and also the 161-st hexagonal number.

It is an amenable number.

51681 is a deficient number, since it is larger than the sum of its proper divisors (31263).

51681 is a wasteful number, since it uses less digits than its factorization.

51681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 140.

The product of its digits is 240, while the sum is 21.

The square root of 51681 is about 227.3345552264. The cubic root of 51681 is about 37.2486298912.

The spelling of 51681 in words is "fifty-one thousand, six hundred eighty-one".