20721709 has 4 divisors (see below), whose sum is σ = 20730816. Its totient is φ = 20712604.

The previous prime is 20721683. The next prime is 20721713. The reversal of 20721709 is 90712702.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20721709 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20721719) 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, 2142 + ... + 6784.

It is an arithmetic number, because the mean of its divisors is an integer number (5182704).

Almost surely, 2^20721709 is an apocalyptic number.

It is an amenable number.

20721709 is a deficient number, since it is larger than the sum of its proper divisors (9107).

20721709 is an equidigital number, since it uses as much as digits as its factorization.

20721709 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9106.

The product of its (nonzero) digits is 1764, while the sum is 28.

The square root of 20721709 is about 4552.1103896984. The cubic root of 20721709 is about 274.6682898531.

The spelling of 20721709 in words is "twenty million, seven hundred twenty-one thousand, seven hundred nine".