21815257 has 2 divisors, whose sum is σ = 21815258. Its totient is φ = 21815256.

The previous prime is 21815251. The next prime is 21815281. The reversal of 21815257 is 75251812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11128896 + 10686361 = 3336^2 + 3269^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21815257 is a prime.

It is not a weakly prime, because it can be changed into another prime (21815251) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10907628 + 10907629.

It is an arithmetic number, because the mean of its divisors is an integer number (10907629).

Almost surely, 2^21815257 is an apocalyptic number.

It is an amenable number.

21815257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21815257 is an equidigital number, since it uses as much as digits as its factorization.

21815257 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5600, while the sum is 31.

The square root of 21815257 is about 4670.6805713943. The cubic root of 21815257 is about 279.4173983034.

The spelling of 21815257 in words is "twenty-one million, eight hundred fifteen thousand, two hundred fifty-seven".