20428721 has 4 divisors (see below), whose sum is σ = 21087744. Its totient is φ = 19769700.

The previous prime is 20428697. The next prime is 20428739. The reversal of 20428721 is 12782402.

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 a de Polignac number, because none of the positive numbers 2^k-20428721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20428621) 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, 329465 + ... + 329526.

It is an arithmetic number, because the mean of its divisors is an integer number (5271936).

Almost surely, 2^20428721 is an apocalyptic number.

It is an amenable number.

20428721 is a deficient number, since it is larger than the sum of its proper divisors (659023).

20428721 is an equidigital number, since it uses as much as digits as its factorization.

20428721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 659022.

The product of its (nonzero) digits is 1792, while the sum is 26.

The square root of 20428721 is about 4519.8142660955. The cubic root of 20428721 is about 273.3676120338.

The spelling of 20428721 in words is "twenty million, four hundred twenty-eight thousand, seven hundred twenty-one".