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391 = 1723

• The smallest n such that \sqrt{n} (here 19.773719933...) starts with 10 (actually, 11) odd digits.

391 has 4 divisors (see below), whose sum is σ = 432. Its totient is φ = 352.

The previous prime is 389. The next prime is 397. The reversal of 391 is 193.

It is a happy number.

391 is nontrivially palindromic in base 9 and base 15.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 391 - 21 = 389 is a prime.

It is a Smith number, since the sum of its digits (13) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

391 is an undulating number in base 9 and base 15.

391 is a lucky number.

It is a plaindrome in base 14.

It is a zygodrome in base 2.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (397) by changing a digit.

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 in 3 ways as a sum of consecutive naturals, for example, 6 + ... + 28.

It is an arithmetic number, because the mean of its divisors is an integer number (108).

391 is the 13-th centered pentagonal number.

391 is a deficient number, since it is larger than the sum of its proper divisors (41).

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

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

The sum of its prime factors is 40.

The product of its digits is 27, while the sum is 13.

The square root of 391 is about 19.7737199333. The cubic root of 391 is about 7.3123828116.

Adding to 391 its sum of digits (13), we get a palindrome (404).

Subtracting from 391 its sum of digits (13), we obtain a triangular number (378 = T27).

The spelling of 391 in words is "three hundred ninety-one", and thus it is an aban number.

Divisors: 1 17 23 391