• 138 can be written using four 4's:

138 has 8 divisors (see below), whose sum is σ = 288. Its totient is φ = 44.

The previous prime is 137. The next prime is 139. The reversal of 138 is 831.

138 is nontrivially palindromic in base 8.

138 is an esthetic number in base 8, because in such base its adjacent digits differ by 1.

It is an interprime number because it is at equal distance from previous prime (137) and next prime (139).

It is a sphenic number, since it is the product of 3 distinct primes.

138 is an admirable number.

It is an Ulam number.

138 is an undulating number in base 8.

It is a Curzon number.

It is a plaindrome in base 7, base 10, base 11, base 14 and base 16.

It is a nialpdrome in base 12, base 13 and base 15.

It is a congruent number.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6 + ... + 17.

It is an arithmetic number, because the mean of its divisors is an integer number (36).

138 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (144).

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

138 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 28.

The product of its digits is 24, while the sum is 12.

The square root of 138 is about 11.7473401245. The cubic root of 138 is about 5.1676492524.

Adding to 138 its reverse (831), we get a palindrome (969).

It can be divided in two parts, 13 and 8, that added together give a triangular number (21 = T_{6}).

The spelling of 138 in words is "one hundred thirty-eight", and thus it is an aban number.

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