• 581 can be written using four 4's:
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: 185 = 5 ⋅37.
It is a cyclic number.
It is an alternating number because its digits alternate between odd and even.
It is a Canada number.
It is a Curzon number.
It is a plaindrome in base 11, base 13, base 15 and base 16.
It is a nialpdrome in base 5.
It is a congruent number.
It is an amenable number.
581 is an equidigital number, since it uses as much as digits as its factorization.
581 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 90.
The square root of 581 is about 24.1039415864. The cubic root of 581 is about 8.3443410089.
Adding to 581 its sum of digits (14), we get a palindrome (595).
The spelling of 581 in words is "five hundred eighty-one", and thus it is an aban number.