21716721 has 16 divisors (see below), whose sum is σ = 34648320. Its totient is φ = 13363920.

The previous prime is 21716713. The next prime is 21716729. The reversal of 21716721 is 12761712.

It is an interprime number because it is at equal distance from previous prime (21716713) and next prime (21716729).

It is not a de Polignac number, because 21716721 - 2³ = 21716713 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21716729) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 30585 + ... + 31286.

It is an arithmetic number, because the mean of its divisors is an integer number (2165520).

Almost surely, 2^21716721 is an apocalyptic number.

It is an amenable number.

21716721 is a deficient number, since it is larger than the sum of its proper divisors (12931599).

21716721 is a wasteful number, since it uses less digits than its factorization.

21716721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 61893 (or 61887 counting only the distinct ones).

The product of its digits is 1176, while the sum is 27.

The square root of 21716721 is about 4660.1202774177. The cubic root of 21716721 is about 278.9960688543.

The spelling of 21716721 in words is "twenty-one million, seven hundred sixteen thousand, seven hundred twenty-one".