21651721 has 16 divisors (see below), whose sum is σ = 26790400. Its totient is φ = 17039808.

The previous prime is 21651709. The next prime is 21651769. The reversal of 21651721 is 12715612.

It is a cyclic number.

It is not a de Polignac number, because 21651721 - 2⁵ = 21651689 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21651701) 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, 20320 + ... + 21358.

It is an arithmetic number, because the mean of its divisors is an integer number (1674400).

Almost surely, 2^21651721 is an apocalyptic number.

It is an amenable number.

21651721 is a deficient number, since it is larger than the sum of its proper divisors (5138679).

21651721 is a wasteful number, since it uses less digits than its factorization.

21651721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1288.

The product of its digits is 840, while the sum is 25.

The square root of 21651721 is about 4653.1409821754. The cubic root of 21651721 is about 278.7174376624.

The spelling of 21651721 in words is "twenty-one million, six hundred fifty-one thousand, seven hundred twenty-one".