1991861 has 2 divisors, whose sum is σ = 1991862. Its totient is φ = 1991860.

The previous prime is 1991849. The next prime is 1991863. The reversal of 1991861 is 1681991.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1705636 + 286225 = 1306^2 + 535^2 .

It is an emirp because it is prime and its reverse (1681991) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 1991861 - 2⁶ = 1991797 is a prime.

Together with 1991863, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1991863) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 995930 + 995931.

It is an arithmetic number, because the mean of its divisors is an integer number (995931).

2^1991861 is an apocalyptic number.

It is an amenable number.

1991861 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991861 is an equidigital number, since it uses as much as digits as its factorization.

1991861 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3888, while the sum is 35.

The square root of 1991861 is about 1411.3330577861. Note that the first 3 decimals coincide. The cubic root of 1991861 is about 125.8209643364.

The spelling of 1991861 in words is "one million, nine hundred ninety-one thousand, eight hundred sixty-one".