21205721 has 4 divisors (see below), whose sum is σ = 21565200. Its totient is φ = 20846244.

The previous prime is 21205703. The next prime is 21205727. The reversal of 21205721 is 12750212.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21205721 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21205693 and 21205702.

It is not an unprimeable number, because it can be changed into a prime (21205727) 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, 179651 + ... + 179768.

It is an arithmetic number, because the mean of its divisors is an integer number (5391300).

Almost surely, 2^21205721 is an apocalyptic number.

It is an amenable number.

21205721 is a deficient number, since it is larger than the sum of its proper divisors (359479).

21205721 is an equidigital number, since it uses as much as digits as its factorization.

21205721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 359478.

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 21205721 is about 4604.9669922813. The cubic root of 21205721 is about 276.7903943299.

Adding to 21205721 its reverse (12750212), we get a palindrome (33955933).

The spelling of 21205721 in words is "twenty-one million, two hundred five thousand, seven hundred twenty-one".