20421721 has 4 divisors (see below), whose sum is σ = 20577744. Its totient is φ = 20265700.

The previous prime is 20421719. The next prime is 20421733. The reversal of 20421721 is 12712402.

It is a happy number.

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 20421721 - 2¹ = 20421719 is a prime.

It is a super-3 number, since 3×20421721³ (a number of 23 digits) contains 333 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20424721) 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, 77815 + ... + 78076.

It is an arithmetic number, because the mean of its divisors is an integer number (5144436).

Almost surely, 2^20421721 is an apocalyptic number.

It is an amenable number.

20421721 is a deficient number, since it is larger than the sum of its proper divisors (156023).

20421721 is a wasteful number, since it uses less digits than its factorization.

20421721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 156022.

The product of its (nonzero) digits is 224, while the sum is 19.

The square root of 20421721 is about 4519.0398316457. The cubic root of 20421721 is about 273.3363848889.

The spelling of 20421721 in words is "twenty million, four hundred twenty-one thousand, seven hundred twenty-one".