123991 has 4 divisors (see below), whose sum is σ = 141712. Its totient is φ = 106272.

The previous prime is 123989. The next prime is 123997. The reversal of 123991 is 199321.

123991 = T₉₈ + T₉₉ + ... + T₁₁₈.

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 123991 - 2¹ = 123989 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is a plaindrome in base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (123997) 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, 8850 + ... + 8863.

It is an arithmetic number, because the mean of its divisors is an integer number (35428).

2¹²³⁹⁹¹ is an apocalyptic number.

123991 is a deficient number, since it is larger than the sum of its proper divisors (17721).

123991 is an equidigital number, since it uses as much as digits as its factorization.

123991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17720.

The product of its digits is 486, while the sum is 25.

The square root of 123991 is about 352.1235578600. The cubic root of 123991 is about 49.8651030502.

The spelling of 123991 in words is "one hundred twenty-three thousand, nine hundred ninety-one".