20721739 has 4 divisors (see below), whose sum is σ = 21281824. Its totient is φ = 20161656.

The previous prime is 20721737. The next prime is 20721749. The reversal of 20721739 is 93712702.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 20721739 - 2¹ = 20721737 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 20721739.

It is not an unprimeable number, because it can be changed into a prime (20721737) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 279987 + ... + 280060.

It is an arithmetic number, because the mean of its divisors is an integer number (5320456).

Almost surely, 2^20721739 is an apocalyptic number.

20721739 is a deficient number, since it is larger than the sum of its proper divisors (560085).

20721739 is an equidigital number, since it uses as much as digits as its factorization.

20721739 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 560084.

The product of its (nonzero) digits is 5292, while the sum is 31.

The square root of 20721739 is about 4552.1136848721. The cubic root of 20721739 is about 274.6684224040.

The spelling of 20721739 in words is "twenty million, seven hundred twenty-one thousand, seven hundred thirty-nine".