21309913 has 2 divisors, whose sum is σ = 21309914. Its totient is φ = 21309912.

The previous prime is 21309907. The next prime is 21309973. The reversal of 21309913 is 31990312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13927824 + 7382089 = 3732^2 + 2717^2 .

It is a cyclic number.

It is not a de Polignac number, because 21309913 - 2⁹ = 21309401 is a prime.

It is a super-2 number, since 2×21309913² = 908224784135138, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21309973) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10654956 + 10654957.

It is an arithmetic number, because the mean of its divisors is an integer number (10654957).

Almost surely, 2^21309913 is an apocalyptic number.

It is an amenable number.

21309913 is a deficient number, since it is larger than the sum of its proper divisors (1).

21309913 is an equidigital number, since it uses as much as digits as its factorization.

21309913 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1458, while the sum is 28.

The square root of 21309913 is about 4616.2661318429. The cubic root of 21309913 is about 277.2429803762.

It can be divided in two parts, 21309 and 913, that added together give a palindrome (22222).

The spelling of 21309913 in words is "twenty-one million, three hundred nine thousand, nine hundred thirteen".