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55 (fifty-five) is the
natural number In mathematics, the natural numbers are the numbers 0, 1, 2, 3, and so on, possibly excluding 0. Some start counting with 0, defining the natural numbers as the non-negative integers , while others start with 1, defining them as the positive in ...
following 54 and preceding 56.


Mathematics

55 is: *the 10th
Fibonacci number In mathematics, the Fibonacci sequence is a Integer sequence, sequence in which each element is the sum of the two elements that precede it. Numbers that are part of the Fibonacci sequence are known as Fibonacci numbers, commonly denoted . Many w ...
and the 10th triangular number, The sum of 55's digits is also 10. *the 5th heptagonal number, the 5th square pyramidal number, and the 4th centered nonagonal number. 55 is also the 19th semiprime and the 15th squarefree semiprime, as well as the 32nd nontotient number (including odd numbers > 1) the 36th arithmetic number, the 38th
composite number A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Accordingly it is a positive integer that has at least one divisor other than 1 and itself. Every positive integer is composite, prime numb ...
(where the abundance of 55 = - 38), the 43rd
deficient number In number theory, a deficient number or defective number is a positive integer for which the sum of divisors of is less than . Equivalently, it is a number for which the sum of proper divisors (or aliquot sum) is less than . For example, th ...
, the 45th trapezoidal number, and the 49th polite number.


Transportation

*In the United States, the National Maximum Speed Law prohibited speed limits higher than from 1974 to 1987. The number ''55'' became a popular shorthand for the 55 mph speed limit. For example, a hand with a pair of fives in Texas hold'em poker is referred to as a "speed limit".


References

{{Integers, zero Integers