20721647 has 4 divisors (see below), whose sum is σ = 21812280. Its totient is φ = 19631016.

The previous prime is 20721637. The next prime is 20721683. The reversal of 20721647 is 74612702.

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 20721647 - 2⁶ = 20721583 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 (20721637) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 545288 + ... + 545325.

It is an arithmetic number, because the mean of its divisors is an integer number (5453070).

Almost surely, 2^20721647 is an apocalyptic number.

20721647 is a deficient number, since it is larger than the sum of its proper divisors (1090633).

20721647 is a wasteful number, since it uses less digits than its factorization.

20721647 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1090632.

The product of its (nonzero) digits is 4704, while the sum is 29.

The square root of 20721647 is about 4552.1035796651. The cubic root of 20721647 is about 274.6680159141.

The spelling of 20721647 in words is "twenty million, seven hundred twenty-one thousand, six hundred forty-seven".