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 2k-20718821 is a prime.
It is a Duffinian number.
It is a congruent number.
Almost surely, 220718821 is an apocalyptic number.
It is an amenable number.
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 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".