20720021 has 4 divisors (see below), whose sum is σ = 23680032. Its totient is φ = 17760012.

The previous prime is 20719981. The next prime is 20720027. The reversal of 20720021 is 12002702.

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 an emirpimes, since its reverse is a distinct semiprime: 12002702 = 2 ⋅6001351.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20720021 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 20719978 and 20720005.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20720027) 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, 1479995 + ... + 1480008.

It is an arithmetic number, because the mean of its divisors is an integer number (5920008).

Almost surely, 2^20720021 is an apocalyptic number.

It is an amenable number.

20720021 is a deficient number, since it is larger than the sum of its proper divisors (2960011).

20720021 is an equidigital number, since it uses as much as digits as its factorization.

20720021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2960010.

The product of its (nonzero) digits is 56, while the sum is 14.

The square root of 20720021 is about 4551.9249774134. The cubic root of 20720021 is about 274.6608314485.

Adding to 20720021 its reverse (12002702), we get a palindrome (32722723).

The spelling of 20720021 in words is "twenty million, seven hundred twenty thousand, twenty-one".