321 is nontrivially palindromic in base 7, base 11 and base 16.
321 is an esthetic number in base 10, 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: 123 = 3 ⋅41.
It is a cyclic number.
It is a D-number.
It is an alternating number because its digits alternate between odd and even.
321 is an undulating number in base 7, base 11 and base 16.
It is a Curzon number.
321 is a lucky number.
It is a straight-line number, since its digits are in arithmetic progression.
It is a plaindrome in base 12, base 14 and base 15.
It is a nialpdrome in base 10.
It is an amenable number.
321 is a wasteful number, since it uses less digits than its factorization.
321 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 110.
The square root of 321 is about 17.9164728672. The cubic root of 321 is about 6.8470212776.
Adding to 321 its reverse (123), we get a palindrome (444).