21087817 has 8 divisors (see below), whose sum is σ = 22187592. Its totient is φ = 20016000.

The previous prime is 21087809. The next prime is 21087827. The reversal of 21087817 is 71878012.

It can be written as a sum of positive squares in 4 ways, for example, as 4999696 + 16088121 = 2236^2 + 4011^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21087817 - 2³ = 21087809 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21087827) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 5434 + ... + 8467.

It is an arithmetic number, because the mean of its divisors is an integer number (2773449).

Almost surely, 2^21087817 is an apocalyptic number.

It is an amenable number.

21087817 is a deficient number, since it is larger than the sum of its proper divisors (1099775).

21087817 is a wasteful number, since it uses less digits than its factorization.

21087817 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 13979.

The product of its (nonzero) digits is 6272, while the sum is 34.

The square root of 21087817 is about 4592.1473190655. The cubic root of 21087817 is about 276.2764549510.

It can be divided in two parts, 21087 and 817, that added together give a square (21904 = 148²).

The spelling of 21087817 in words is "twenty-one million, eighty-seven thousand, eight hundred seventeen".