20721021 has 4 divisors (see below), whose sum is σ = 27628032. Its totient is φ = 13814012.

The previous prime is 20721013. The next prime is 20721037. The reversal of 20721021 is 12012702.

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 a cyclic number.

It is not a de Polignac number, because 20721021 - 2³ = 20721013 is a prime.

It is a super-2 number, since 2×20721021² = 858721422564882, which contains 22 as substring.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20721721) 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, 3453501 + ... + 3453506.

It is an arithmetic number, because the mean of its divisors is an integer number (6907008).

Almost surely, 2^20721021 is an apocalyptic number.

It is an amenable number.

20721021 is a deficient number, since it is larger than the sum of its proper divisors (6907011).

20721021 is an equidigital number, since it uses as much as digits as its factorization.

20721021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6907010.

The product of its (nonzero) digits is 56, while the sum is 15.

The square root of 20721021 is about 4552.0348197262. The cubic root of 20721021 is about 274.6652499835.

Adding to 20721021 its reverse (12012702), we get a palindrome (32733723).

The spelling of 20721021 in words is "twenty million, seven hundred twenty-one thousand, twenty-one".