991221 has 24 divisors (see below), whose sum is σ = 1679904. Its totient is φ = 514080.

The previous prime is 991217. The next prime is 991223. The reversal of 991221 is 122199.

It is not a de Polignac number, because 991221 - 2² = 991217 is a prime.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 991191 and 991200.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 991221.

It is not an unprimeable number, because it can be changed into a prime (991223) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 1311 + ... + 1923.

It is an arithmetic number, because the mean of its divisors is an integer number (69996).

2⁹⁹¹²²¹ is an apocalyptic number.

It is an amenable number.

991221 is a deficient number, since it is larger than the sum of its proper divisors (688683).

991221 is a wasteful number, since it uses less digits than its factorization.

991221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 641 (or 634 counting only the distinct ones).

The product of its digits is 324, while the sum is 24.

The square root of 991221 is about 995.6008236236. The cubic root of 991221 is about 99.7065061228.

The spelling of 991221 in words is "nine hundred ninety-one thousand, two hundred twenty-one".