21757 has 2 divisors, whose sum is σ = 21758. Its totient is φ = 21756.

The previous prime is 21751. The next prime is 21767. The reversal of 21757 is 75712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21316 + 441 = 146^2 + 21^2 .

It is a cyclic number.

It is not a de Polignac number, because 21757 - 2¹¹ = 19709 is a prime.

It is a Chen prime.

It is the 148-th Hogben number.

21757 is a lucky number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21751) by changing a digit.

It is a nontrivial repunit in base 147.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10878 + 10879.

It is an arithmetic number, because the mean of its divisors is an integer number (10879).

2²¹⁷⁵⁷ is an apocalyptic number.

It is an amenable number.

21757 is a deficient number, since it is larger than the sum of its proper divisors (1).

21757 is an equidigital number, since it uses as much as digits as its factorization.

21757 is an evil number, because the sum of its binary digits is even.

The product of its digits is 490, while the sum is 22.

The square root of 21757 is about 147.5025423510. The cubic root of 21757 is about 27.9168451265.

It can be divided in two parts, 2175 and 7, that multiplied together give a triangular number (15225 = T₁₇₄).

The spelling of 21757 in words is "twenty-one thousand, seven hundred fifty-seven".