21639397 has 4 divisors (see below), whose sum is σ = 23303980. Its totient is φ = 19974816.

The previous prime is 21639383. The next prime is 21639427. The reversal of 21639397 is 79393612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 13793796 + 7845601 = 3714^2 + 2801^2 .

It is a cyclic number.

It is not a de Polignac number, because 21639397 - 2⁷ = 21639269 is a prime.

It is a Duffinian number.

It is a plaindrome in base 11.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21639337) 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 in 3 ways as a sum of consecutive naturals, for example, 832272 + ... + 832297.

It is an arithmetic number, because the mean of its divisors is an integer number (5825995).

Almost surely, 2^21639397 is an apocalyptic number.

It is an amenable number.

21639397 is a deficient number, since it is larger than the sum of its proper divisors (1664583).

21639397 is a wasteful number, since it uses less digits than its factorization.

21639397 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1664582.

The product of its digits is 61236, while the sum is 40.

The square root of 21639397 is about 4651.8165269065. The cubic root of 21639397 is about 278.6645463223.

The spelling of 21639397 in words is "twenty-one million, six hundred thirty-nine thousand, three hundred ninety-seven".