138991 has 4 divisors (see below), whose sum is σ = 140184. Its totient is φ = 137800.

The previous prime is 138977. The next prime is 139021. The reversal of 138991 is 199831.

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 138991 - 2⁵ = 138959 is a prime.

It is a Duffinian number.

138991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (138191) 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, 400 + ... + 661.

It is an arithmetic number, because the mean of its divisors is an integer number (35046).

2¹³⁸⁹⁹¹ is an apocalyptic number.

138991 is a deficient number, since it is larger than the sum of its proper divisors (1193).

138991 is a wasteful number, since it uses less digits than its factorization.

138991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1192.

The product of its digits is 1944, while the sum is 31.

The square root of 138991 is about 372.8149675107. The cubic root of 138991 is about 51.7998966422.

The spelling of 138991 in words is "one hundred thirty-eight thousand, nine hundred ninety-one".