1991391 has 4 divisors (see below), whose sum is σ = 2655192. Its totient is φ = 1327592.

The previous prime is 1991389. The next prime is 1991413. The reversal of 1991391 is 1931991.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 1991391 - 2¹ = 1991389 is a prime.

It is a super-2 number, since 2×1991391² = 7931276229762, which contains 22 as substring.

It is a D-number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991351) 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, 331896 + ... + 331901.

It is an arithmetic number, because the mean of its divisors is an integer number (663798).

2^1991391 is an apocalyptic number.

1991391 is the 893-rd centered pentagonal number.

1991391 is a deficient number, since it is larger than the sum of its proper divisors (663801).

1991391 is an equidigital number, since it uses as much as digits as its factorization.

1991391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 663800.

The product of its digits is 2187, while the sum is 33.

The square root of 1991391 is about 1411.1665387189. The cubic root of 1991391 is about 125.8110673096.

The spelling of 1991391 in words is "one million, nine hundred ninety-one thousand, three hundred ninety-one".