21171721 has 4 divisors (see below), whose sum is σ = 21381444. Its totient is φ = 20962000.

The previous prime is 21171713. The next prime is 21171727. The reversal of 21171721 is 12717112.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 18147600 + 3024121 = 4260^2 + 1739^2 .

It is a cyclic number.

It is not a de Polignac number, because 21171721 - 2³ = 21171713 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21171692 and 21171701.

It is not an unprimeable number, because it can be changed into a prime (21171727) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 104710 + ... + 104911.

It is an arithmetic number, because the mean of its divisors is an integer number (5345361).

Almost surely, 2^21171721 is an apocalyptic number.

It is an amenable number.

21171721 is a deficient number, since it is larger than the sum of its proper divisors (209723).

21171721 is a wasteful number, since it uses less digits than its factorization.

21171721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 209722.

The product of its digits is 196, while the sum is 22.

The square root of 21171721 is about 4601.2738453607. The cubic root of 21171721 is about 276.6423854112.

Adding to 21171721 its reverse (12717112), we get a palindrome (33888833).

The spelling of 21171721 in words is "twenty-one million, one hundred seventy-one thousand, seven hundred twenty-one".