20721557 has 4 divisors (see below), whose sum is σ = 20733420. Its totient is φ = 20709696.

The previous prime is 20721553. The next prime is 20721583. The reversal of 20721557 is 75512702.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 12013156 + 8708401 = 3466^2 + 2951^2 .

It is a cyclic number.

It is not a de Polignac number, because 20721557 - 2² = 20721553 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 (20721553) 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, 2738 + ... + 6995.

It is an arithmetic number, because the mean of its divisors is an integer number (5183355).

Almost surely, 2^20721557 is an apocalyptic number.

It is an amenable number.

20721557 is a deficient number, since it is larger than the sum of its proper divisors (11863).

20721557 is an equidigital number, since it uses as much as digits as its factorization.

20721557 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 11862.

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

The square root of 20721557 is about 4552.0936941148. The cubic root of 20721557 is about 274.6676182598.

The spelling of 20721557 in words is "twenty million, seven hundred twenty-one thousand, five hundred fifty-seven".