21506687 has 4 divisors (see below), whose sum is σ = 21616056. Its totient is φ = 21397320.

The previous prime is 21506669. The next prime is 21506707. The reversal of 21506687 is 78660512.

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 21506687 - 2⁸ = 21506431 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 (21506627) 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, 54389 + ... + 54782.

It is an arithmetic number, because the mean of its divisors is an integer number (5404014).

Almost surely, 2^21506687 is an apocalyptic number.

21506687 is a deficient number, since it is larger than the sum of its proper divisors (109369).

21506687 is a wasteful number, since it uses less digits than its factorization.

21506687 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 109368.

The product of its (nonzero) digits is 20160, while the sum is 35.

The square root of 21506687 is about 4637.5302694430. The cubic root of 21506687 is about 278.0937135535.

The spelling of 21506687 in words is "twenty-one million, five hundred six thousand, six hundred eighty-seven".