Numbers divisible by their own digit sum.
A Harshad number (or Niven number) is an integer that is divisible by the sum of its digits. Since digit sums are completely base-dependent, the set of Harshad numbers is totally different in every base.
For example, 12 is Harshad in base 10 because 1+2=3 and 12/3=4. But in base 7, 12 is written as 15 (digits 1 and 5), digit sum 6, and 12/6=2, so it is also Harshad. Other numbers are Harshad in one base but not another.
Numbers in green are Harshad numbers.
Numbers in red are palindromic Harshad numbers.
Numbers in gray are not Harshad.