123811991 has 4 divisors (see below), whose sum is σ = 128081400. Its totient is φ = 119542584.

The previous prime is 123811951. The next prime is 123812011. The reversal of 123811991 is 199118321.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199118321 = 29 ⋅6866149.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-123811991 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (123811951) 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, 2134661 + ... + 2134718.

It is an arithmetic number, because the mean of its divisors is an integer number (32020350).

Almost surely, 2^123811991 is an apocalyptic number.

123811991 is a deficient number, since it is larger than the sum of its proper divisors (4269409).

123811991 is an equidigital number, since it uses as much as digits as its factorization.

123811991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4269408.

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

The square root of 123811991 is about 11127.0836700368. The cubic root of 123811991 is about 498.4109431467.

The spelling of 123811991 in words is "one hundred twenty-three million, eight hundred eleven thousand, nine hundred ninety-one".