21687491 has 4 divisors (see below), whose sum is σ = 24785712. Its totient is φ = 18589272.

The previous prime is 21687473. The next prime is 21687499. The reversal of 21687491 is 19478612.

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

It is a cyclic number.

It is not a de Polignac number, because 21687491 - 2¹⁸ = 21425347 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21687499) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1549100 + ... + 1549113.

It is an arithmetic number, because the mean of its divisors is an integer number (6196428).

Almost surely, 2^21687491 is an apocalyptic number.

21687491 is a deficient number, since it is larger than the sum of its proper divisors (3098221).

21687491 is an equidigital number, since it uses as much as digits as its factorization.

21687491 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3098220.

The product of its digits is 24192, while the sum is 38.

The square root of 21687491 is about 4656.9830362586. The cubic root of 21687491 is about 278.8708394417.

The spelling of 21687491 in words is "twenty-one million, six hundred eighty-seven thousand, four hundred ninety-one".