21639813 has 16 divisors (see below), whose sum is σ = 31221120. Its totient is φ = 13253184.

The previous prime is 21639809. The next prime is 21639823. The reversal of 21639813 is 31893612.

21639813 is nontrivially palindromic in base 4.

It is not a de Polignac number, because 21639813 - 2² = 21639809 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21639823) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 7720 + ... + 10142.

It is an arithmetic number, because the mean of its divisors is an integer number (1951320).

Almost surely, 2^21639813 is an apocalyptic number.

It is an amenable number.

21639813 is a deficient number, since it is larger than the sum of its proper divisors (9581307).

21639813 is a wasteful number, since it uses less digits than its factorization.

21639813 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2668.

The product of its digits is 7776, while the sum is 33.

The square root of 21639813 is about 4651.8612404069. The cubic root of 21639813 is about 278.6663320115. Note that the first 3 decimals are identical.

The spelling of 21639813 in words is "twenty-one million, six hundred thirty-nine thousand, eight hundred thirteen".