• 84 can be written using four 4's:
84 is nontrivially palindromic in base 11 and base 13.
84 is an esthetic number in base 16, because in such base its adjacent digits differ by 1.
It is a tau number, because it is divible by the number of its divisors (12).
84 is an admirable number.
It is a O'Halloran number.
84 is a nontrivial repdigit in base 11 and base 13.
It is a plaindrome in base 8, base 11, base 13 and base 15.
It is a nialpdrome in base 4, base 6, base 10, base 11, base 12, base 13, base 14 and base 16.
It is a zygodrome in base 11 and base 13.
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 84.
It is the 7-th tetrahedral number.
It is an amenable number.
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
84 is a wasteful number, since it uses less digits than its factorization.
84 is an odious number, because the sum of its binary digits is odd.
The square root of 84 is about 9.1651513899. The cubic root of 84 is about 4.3795191399.
Subtracting from 84 its reverse (48), we obtain a triangular number (36 = T8).
The spelling of 84 in words is "eighty-four", and thus it is an aban number.