• 840 can be written using four 4's:
It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.
840 is an idoneal number.
It is a straight-line number, since its digits are in arithmetic progression.
It is a plaindrome in base 9 and base 16.
It is a nialpdrome in base 10 and base 14.
It is a zygodrome in base 9.
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 840.
It is an unprimeable number.
840 is a highly composite number, because it has more divisors than any smaller number.
840 is a superabundant number, because it has a larger abundancy index than any smaller number.
2840 is an apocalyptic number.
It is an amenable number.
It is a practical number, because each smaller number is the sum of distinct divisors of 840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1440).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
840 is a wasteful number, since it uses less digits than its factorization.
840 is an evil number, because the sum of its binary digits is even.
The square root of 840 is about 28.9827534924. The cubic root of 840 is about 9.4353879606.
Subtracting from 840 its sum of digits (12), we obtain a palindrome (828).
Subtracting from 840 its product of nonzero digits (32), we obtain a palindrome (808).
Adding to 840 its reverse (48), we get a palindrome (888).
The spelling of 840 in words is "eight hundred forty", and thus it is an aban number.