991721 has 4 divisors (see below), whose sum is σ = 1023744. Its totient is φ = 959700.

The previous prime is 991717. The next prime is 991723. The reversal of 991721 is 127199.

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, and also an emirpimes, since its reverse is a distinct semiprime: 127199 = 311 ⋅409.

It is a cyclic number.

It is not a de Polignac number, because 991721 - 2² = 991717 is a prime.

It is not an unprimeable number, because it can be changed into a prime (991723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 15965 + ... + 16026.

It is an arithmetic number, because the mean of its divisors is an integer number (255936).

2⁹⁹¹⁷²¹ is an apocalyptic number.

It is an amenable number.

991721 is a deficient number, since it is larger than the sum of its proper divisors (32023).

991721 is a wasteful number, since it uses less digits than its factorization.

991721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 32022.

The product of its digits is 1134, while the sum is 29.

The square root of 991721 is about 995.8518966192. The cubic root of 991721 is about 99.7232682350.

The spelling of 991721 in words is "nine hundred ninety-one thousand, seven hundred twenty-one".