21751 has 2 divisors, whose sum is σ = 21752. Its totient is φ = 21750.

The previous prime is 21739. The next prime is 21757. The reversal of 21751 is 15712.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21751 is a prime.

21751 is a lucky number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21757) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10875 + 10876.

It is an arithmetic number, because the mean of its divisors is an integer number (10876).

2²¹⁷⁵¹ is an apocalyptic number.

21751 is a deficient number, since it is larger than the sum of its proper divisors (1).

21751 is an equidigital number, since it uses as much as digits as its factorization.

21751 is an evil number, because the sum of its binary digits is even.

The product of its digits is 70, while the sum is 16.

The square root of 21751 is about 147.4822023161. The cubic root of 21751 is about 27.9142786502.

The spelling of 21751 in words is "twenty-one thousand, seven hundred fifty-one".