21756257 has 2 divisors, whose sum is σ = 21756258. Its totient is φ = 21756256.

The previous prime is 21756239. The next prime is 21756269. The reversal of 21756257 is 75265712.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21077281 + 678976 = 4591^2 + 824^2 .

It is a cyclic number.

It is not a de Polignac number, because 21756257 - 2⁶ = 21756193 is a prime.

It is not a weakly prime, because it can be changed into another prime (21756157) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10878128 + 10878129.

It is an arithmetic number, because the mean of its divisors is an integer number (10878129).

Almost surely, 2^21756257 is an apocalyptic number.

It is an amenable number.

21756257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21756257 is an equidigital number, since it uses as much as digits as its factorization.

21756257 is an evil number, because the sum of its binary digits is even.

The product of its digits is 29400, while the sum is 35.

The square root of 21756257 is about 4664.3602991193. The cubic root of 21756257 is about 279.1652733675.

The spelling of 21756257 in words is "twenty-one million, seven hundred fifty-six thousand, two hundred fifty-seven".