• 55 can be written using four 4's:

55 has 4 divisors (see below), whose sum is σ = 72. Its totient is φ = 40.

The previous prime is 53. The next prime is 59.

55 = T_{2} + T_{3} + ... +
T_{6}.

55 = 1^{2} + 2^{2} + ... + 5^{2}.

It is the 9-th Fibonacci number F_{9}.

55 is nontrivially palindromic in base 4, base 6 and base 10.

55 is an esthetic number in base 5, base 8 and base 13, because in such bases its adjacent digits differ by 1.

55 is a nontrivial binomial coefficient, being equal to C(11, 2).

It is a semiprime because it is the product of two primes.

It is not a de Polignac number, because 55 - 2^{1} = 53 is a prime.

It is a nude number because it is divisible by every one of its digits.

It is a Duffinian number.

55 is an undulating number in base 4 and base 6.

55 is a nontrivial repdigit in base 10.

It is a plaindrome in base 8, base 10, base 12, base 14, base 15 and base 16.

It is a nialpdrome in base 5, base 9, base 10, base 11 and base 13.

It is a zygodrome in base 10.

It is a congruent number.

It is a Kaprekar number, because its square (3025) can be partitioned into two parts whose sum is 55.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is an upside-down number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 10.

It is an arithmetic number, because the mean of its divisors is an integer number (18).

55 is the 10-th triangular number and also the 5-th heptagonal number.

55 is the 4-th centered nonagonal number.

55 is a deficient number, since it is larger than the sum of its proper divisors (17).

55 is a wasteful number, since it uses less digits than its factorization.

55 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 16.

The product of its digits is 25, while the sum is 10.

The square root of 55 is about 7.4161984871. The cubic root of 55 is about 3.8029524608.

Subtracting from 55 its sum of digits (10), we obtain a triangular number (45 = T_{9}).

The spelling of 55 in words is "fifty-five", and thus it is an aban number, an oban number, and an uban number.

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