221664721 has 4 divisors (see below), whose sum is σ = 222482944. Its totient is φ = 220846500.

The previous prime is 221664713. The next prime is 221664733. The reversal of 221664721 is 127466122.

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 not a de Polignac number, because 221664721 - 2³ = 221664713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (221664221) 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, 408705 + ... + 409246.

It is an arithmetic number, because the mean of its divisors is an integer number (55620736).

Almost surely, 2^221664721 is an apocalyptic number.

It is an amenable number.

221664721 is a deficient number, since it is larger than the sum of its proper divisors (818223).

221664721 is an equidigital number, since it uses as much as digits as its factorization.

221664721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 818222.

The product of its digits is 8064, while the sum is 31.

The square root of 221664721 is about 14888.4089479031. The cubic root of 221664721 is about 605.1999166291.

The spelling of 221664721 in words is "two hundred twenty-one million, six hundred sixty-four thousand, seven hundred twenty-one".