• 381 can be written using four 4's:
381 is nontrivially palindromic in base 2 and base 8.
381 is an esthetic number in base 13, because in such base its adjacent digits differ by 1.
It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 183 = 3 ⋅61.
It is a D-number.
It is an alternating number because its digits alternate between odd and even.
It is the 20-th Hogben number.
It is a Duffinian number.
381 is an undulating number in base 8.
It is a plaindrome in base 12, base 13 and base 16.
It is a congruent number.
It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 381.
It is an amenable number.
381 is a wasteful number, since it uses less digits than its factorization.
381 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 130.
The square root of 381 is about 19.5192212959. The cubic root of 381 is about 7.2495045242.
Adding to 381 its sum of digits (12), we get a palindrome (393).
The spelling of 381 in words is "three hundred eighty-one", and thus it is an aban number.