20718821 has 4 divisors (see below), whose sum is σ = 20728032. Its totient is φ = 20709612.

The previous prime is 20718809. The next prime is 20718823. The reversal of 20718821 is 12881702.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20718821 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 (20718823) 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, 1256 + ... + 6558.

It is an arithmetic number, because the mean of its divisors is an integer number (5182008).

Almost surely, 2^20718821 is an apocalyptic number.

It is an amenable number.

20718821 is a deficient number, since it is larger than the sum of its proper divisors (9211).

20718821 is an equidigital number, since it uses as much as digits as its factorization.

20718821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9210.

The product of its (nonzero) digits is 1792, while the sum is 29.

The square root of 20718821 is about 4551.7931631391. The cubic root of 20718821 is about 274.6555290189.

The spelling of 20718821 in words is "twenty million, seven hundred eighteen thousand, eight hundred twenty-one".