20810721 has 4 divisors (see below), whose sum is σ = 27747632. Its totient is φ = 13873812.

The previous prime is 20810717. The next prime is 20810731. The reversal of 20810721 is 12701802.

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.

It is not a de Polignac number, because 20810721 - 2² = 20810717 is a prime.

It is a super-2 number, since 2×20810721² = 866172217079682, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (20810731) 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, 3468451 + ... + 3468456.

It is an arithmetic number, because the mean of its divisors is an integer number (6936908).

Almost surely, 2^20810721 is an apocalyptic number.

It is an amenable number.

20810721 is a deficient number, since it is larger than the sum of its proper divisors (6936911).

20810721 is an equidigital number, since it uses as much as digits as its factorization.

20810721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6936910.

The product of its (nonzero) digits is 224, while the sum is 21.

The square root of 20810721 is about 4561.8769163580. The cubic root of 20810721 is about 275.0610156622.

The spelling of 20810721 in words is "twenty million, eight hundred ten thousand, seven hundred twenty-one".