21309257 has 2 divisors, whose sum is σ = 21309258. Its totient is φ = 21309256.

The previous prime is 21309221. The next prime is 21309259. The reversal of 21309257 is 75290312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17131321 + 4177936 = 4139^2 + 2044^2 .

It is a cyclic number.

It is not a de Polignac number, because 21309257 - 2⁶ = 21309193 is a prime.

Together with 21309259, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_1348283 and since 21309257 and 1348283 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (21309259) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10654628 + 10654629.

It is an arithmetic number, because the mean of its divisors is an integer number (10654629).

Almost surely, 2^21309257 is an apocalyptic number.

It is an amenable number.

21309257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21309257 is an equidigital number, since it uses as much as digits as its factorization.

21309257 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3780, while the sum is 29.

The square root of 21309257 is about 4616.1950782002. The cubic root of 21309257 is about 277.2401354833.

Adding to 21309257 its reverse (75290312), we get a palindrome (96599569).

The spelling of 21309257 in words is "twenty-one million, three hundred nine thousand, two hundred fifty-seven".