21720557 has 8 divisors (see below), whose sum is σ = 21991200. Its totient is φ = 21451824.

The previous prime is 21720511. The next prime is 21720563. The reversal of 21720557 is 75502712.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21720557 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 (21740557) 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, 42419 + ... + 42927.

It is an arithmetic number, because the mean of its divisors is an integer number (2748900).

Almost surely, 2^21720557 is an apocalyptic number.

It is an amenable number.

21720557 is a deficient number, since it is larger than the sum of its proper divisors (270643).

21720557 is a wasteful number, since it uses less digits than its factorization.

21720557 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 955.

The product of its (nonzero) digits is 4900, while the sum is 29.

The square root of 21720557 is about 4660.5318366041. The cubic root of 21720557 is about 279.0124949976.

The spelling of 21720557 in words is "twenty-one million, seven hundred twenty thousand, five hundred fifty-seven".