400 has 15 divisors (see below), whose sum is σ = 961. Its totient is φ = 160.

The previous prime is 397. The next prime is 401. The reversal of 400 is 4.

400 = T_{19} + T_{20}.

The square root of 400 is 20.

It is a perfect power (a square), and thus also a powerful number.

400 is nontrivially palindromic in base 3, base 7 and base 9.

It can be written as a sum of positive squares in only one way, i.e., 144 + 256 = 12^2 + 16^2 .

It is a Harshad number since it is a multiple of its sum of digits (4).

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.

It is not an unprimeable number, because it can be changed into a prime (401) by changing a digit.

It is a nontrivial repunit in base 7.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 78 + ... + 82.

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

400 is an abundant number, since it is smaller than the sum of its proper divisors (561).

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 sum of its prime factors is 18 (or 7 counting only the distinct ones).

The product of its (nonzero) digits is 4, while the sum is 4.

The cubic root of 400 is about 7.3680629973.

The spelling of 400 in words is "four hundred", and thus it is an aban number and an iban number.

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