21135721 has 8 divisors (see below), whose sum is σ = 21453824. Its totient is φ = 20820240.

The previous prime is 21135713. The next prime is 21135731. The reversal of 21135721 is 12753112.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21135721 - 2³ = 21135713 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21135692 and 21135701.

It is not an unprimeable number, because it can be changed into a prime (21135731) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 20191 + ... + 21211.

It is an arithmetic number, because the mean of its divisors is an integer number (2681728).

Almost surely, 2^21135721 is an apocalyptic number.

It is an amenable number.

21135721 is a deficient number, since it is larger than the sum of its proper divisors (318103).

21135721 is a wasteful number, since it uses less digits than its factorization.

21135721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1311.

The product of its digits is 420, while the sum is 22.

The square root of 21135721 is about 4597.3602208224. The cubic root of 21135721 is about 276.4854972681.

Adding to 21135721 its reverse (12753112), we get a palindrome (33888833).

The spelling of 21135721 in words is "twenty-one million, one hundred thirty-five thousand, seven hundred twenty-one".