21687157 has 4 divisors (see below), whose sum is σ = 22435020. Its totient is φ = 20939296.

The previous prime is 21687119. The next prime is 21687191. The reversal of 21687157 is 75178612.

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 8444836 + 13242321 = 2906^2 + 3639^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21687157 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 (21687857) 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, 373888 + ... + 373945.

It is an arithmetic number, because the mean of its divisors is an integer number (5608755).

Almost surely, 2^21687157 is an apocalyptic number.

It is an amenable number.

21687157 is a deficient number, since it is larger than the sum of its proper divisors (747863).

21687157 is an equidigital number, since it uses as much as digits as its factorization.

21687157 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 747862.

The product of its digits is 23520, while the sum is 37.

The square root of 21687157 is about 4656.9471759942. The cubic root of 21687157 is about 278.8694078432.

The spelling of 21687157 in words is "twenty-one million, six hundred eighty-seven thousand, one hundred fifty-seven".