221721 has 4 divisors (see below), whose sum is σ = 295632. Its totient is φ = 147812.

The previous prime is 221719. The next prime is 221723. The reversal of 221721 is 127122.

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 an interprime number because it is at equal distance from previous prime (221719) and next prime (221723).

It is a cyclic number.

It is not a de Polignac number, because 221721 - 2¹ = 221719 is a prime.

It is a D-number.

It is a self number, because there is not a number n which added to its sum of digits gives 221721.

It is not an unprimeable number, because it can be changed into a prime (221723) 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, 36951 + ... + 36956.

It is an arithmetic number, because the mean of its divisors is an integer number (73908).

2²²¹⁷²¹ is an apocalyptic number.

It is an amenable number.

221721 is a deficient number, since it is larger than the sum of its proper divisors (73911).

221721 is an equidigital number, since it uses as much as digits as its factorization.

221721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 73910.

The product of its digits is 56, while the sum is 15.

The square root of 221721 is about 470.8725942333. The cubic root of 221721 is about 60.5251130859.

Adding to 221721 its reverse (127122), we get a palindrome (348843).

The spelling of 221721 in words is "two hundred twenty-one thousand, seven hundred twenty-one", and thus it is an iban number.