21387917 has 4 divisors (see below), whose sum is σ = 21448860. Its totient is φ = 21326976.

The previous prime is 21387893. The next prime is 21387929. The reversal of 21387917 is 71978312.

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 14092516 + 7295401 = 3754^2 + 2701^2 .

It is a cyclic number.

It is not a de Polignac number, because 21387917 - 2⁸ = 21387661 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21387617) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 29942 + ... + 30647.

It is an arithmetic number, because the mean of its divisors is an integer number (5362215).

Almost surely, 2^21387917 is an apocalyptic number.

It is an amenable number.

21387917 is a deficient number, since it is larger than the sum of its proper divisors (60943).

21387917 is an equidigital number, since it uses as much as digits as its factorization.

21387917 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 60942.

The product of its digits is 21168, while the sum is 38.

The square root of 21387917 is about 4624.7072339771. The cubic root of 21387917 is about 277.5808470440.

The spelling of 21387917 in words is "twenty-one million, three hundred eighty-seven thousand, nine hundred seventeen".