21515957 has 2 divisors, whose sum is σ = 21515958. Its totient is φ = 21515956.

The previous prime is 21515917. The next prime is 21515983. The reversal of 21515957 is 75951512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15295921 + 6220036 = 3911^2 + 2494^2 .

It is a cyclic number.

It is not a de Polignac number, because 21515957 - 2⁸ = 21515701 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21515917) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10757978 + 10757979.

It is an arithmetic number, because the mean of its divisors is an integer number (10757979).

Almost surely, 2^21515957 is an apocalyptic number.

It is an amenable number.

21515957 is a deficient number, since it is larger than the sum of its proper divisors (1).

21515957 is an equidigital number, since it uses as much as digits as its factorization.

21515957 is an evil number, because the sum of its binary digits is even.

The product of its digits is 15750, while the sum is 35.

The square root of 21515957 is about 4638.5296161607. The cubic root of 21515957 is about 278.1336632743.

The spelling of 21515957 in words is "twenty-one million, five hundred fifteen thousand, nine hundred fifty-seven".