1891991 has 2 divisors, whose sum is σ = 1891992. Its totient is φ = 1891990.

The previous prime is 1891987. The next prime is 1891997. The reversal of 1891991 is 1991981.

It is a happy number.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 1891991 - 2² = 1891987 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1891997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 945995 + 945996.

It is an arithmetic number, because the mean of its divisors is an integer number (945996).

2^1891991 is an apocalyptic number.

1891991 is a deficient number, since it is larger than the sum of its proper divisors (1).

1891991 is an equidigital number, since it uses as much as digits as its factorization.

1891991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5832, while the sum is 38.

The square root of 1891991 is about 1375.4966375822. The cubic root of 1891991 is about 123.6819589910.

It can be divided in two parts, 1891 and 991, that added together give a palindrome (2882).

The spelling of 1891991 in words is "one million, eight hundred ninety-one thousand, nine hundred ninety-one".