• 181 can be written using four 4's:

181 has 2 divisors, whose sum is σ = 182. Its totient is φ = 180.

The previous prime is 179. The next prime is 191.

181 is nontrivially palindromic in base 10 and base 12.

It is the 6-th star number.

181 is an esthetic number in base 14, because in such base its adjacent digits differ by 1.

It is an a-pointer prime, because the next prime (191) can be obtained adding 181 to its sum of digits (10).

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 100 + 81 = 10^2 + 9^2 .

It is a palprime.

It is a cyclic number.

It is not a de Polignac number, because 181 - 2^{1} = 179 is a prime.

It is a super-2 number, since 2×181^{2} = 65522, which contains 22 as substring.

Together with 179, it forms a pair of twin primes.

It is a Chen prime.

It is an alternating number because its digits alternate between odd and even.

181 is a strobogrammatic number because it is the same when read upside-down.

181 is an undulating number in base 10 and base 12.

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

It is a nialpdrome in base 9, base 15 and base 16.

It is a congruent number.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 90 + 91.

It is an arithmetic number, because the mean of its divisors is an integer number (91).

181 is the 10-th centered square number and also the 9-th centered pentagonal number.

It is an amenable number.

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

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

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

The product of its digits is 8, while the sum is 10.

The square root of 181 is about 13.4536240471. The cubic root of 181 is about 5.6566528258.

Adding to 181 its sum of digits (10), we get a palindrome (191).

Subtracting from 181 its sum of digits (10), we obtain a palindrome (171).

The spelling of 181 in words is "one hundred eighty-one", and thus it is an aban number.

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