1991999 has 2 divisors, whose sum is σ = 1992000. Its totient is φ = 1991998.

The previous prime is 1991993. The next prime is 1992031. The reversal of 1991999 is 9991991.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1991999 is a prime.

It is a Sophie Germain prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1991993) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 995999 + 996000.

It is an arithmetic number, because the mean of its divisors is an integer number (996000).

2^1991999 is an apocalyptic number.

1991999 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991999 is an equidigital number, since it uses as much as digits as its factorization.

1991999 is an evil number, because the sum of its binary digits is even.

The product of its digits is 59049, while the sum is 47.

The square root of 1991999 is about 1411.3819468875. The cubic root of 1991999 is about 125.8238699763.

The spelling of 1991999 in words is "one million, nine hundred ninety-one thousand, nine hundred ninety-nine".