The square root of 400 is 20.
400 is nontrivially palindromic in base 3, base 7 and base 9.
It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.
It is an Ulam number.
It is a Duffinian number.
400 is an undulating number in base 9.
400 is a nontrivial repdigit in base 7.
It is a plaindrome in base 7, base 11 and base 13.
It is a nialpdrome in base 5, base 7, base 8 and base 10.
It is a zygodrome in base 3 and base 7.
It is a self number, because there is not a number n which added to its sum of digits gives 400.
400 is a gapful number since it is divisible by the number (40) formed by its first and last digit.
400 is the 20-th square number.
It is an amenable number.
It is a practical number, because each smaller number is the sum of distinct divisors of 400
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
400 is a wasteful number, since it uses less digits than its factorization.
400 is an odious number, because the sum of its binary digits is odd.
The cubic root of 400 is about 7.3680629973.