21087985 has 4 divisors (see below), whose sum is σ = 25305588. Its totient is φ = 16870384.

The previous prime is 21087971. The next prime is 21087991. The reversal of 21087985 is 58978012.

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 7420176 + 13667809 = 2724^2 + 3697^2 .

It is a cyclic number.

It is not a de Polignac number, because 21087985 - 2⁵ = 21087953 is a prime.

It is a super-2 number, since 2×21087985² = 889406222720450, which contains 22 as substring.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2108794 + ... + 2108803.

It is an arithmetic number, because the mean of its divisors is an integer number (6326397).

Almost surely, 2^21087985 is an apocalyptic number.

It is an amenable number.

21087985 is a deficient number, since it is larger than the sum of its proper divisors (4217603).

21087985 is an equidigital number, since it uses as much as digits as its factorization.

21087985 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4217602.

The product of its (nonzero) digits is 40320, while the sum is 40.

The square root of 21087985 is about 4592.1656111251. The cubic root of 21087985 is about 276.2771886182.

The spelling of 21087985 in words is "twenty-one million, eighty-seven thousand, nine hundred eighty-five".