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A repeating decimal or recurring decimal is a decimal representation of a number whose digits are eventually periodic (that is, after some place, the same sequence of digits is repeated forever); if this sequence consists only of zeros (that is if there is only a finite number of nonzero digits), the decimal is said to be terminating, and is not considered as repeating.
A repeating decimal is an infinite decimal that, after some place, repeats indefinitely the same sequence of digits (e.g., 5.123144144144144... = 5.123 144). [4] An infinite decimal represents a rational number, the quotient of two integers, if and only if it is a repeating decimal or has a finite number of non-zero digits.
Also the converse is true: The decimal expansion of a rational number is either finite, or endlessly repeating. Finite decimal representations can also be seen as a special case of infinite repeating decimal representations. For example, 36 ⁄ 25 = 1.44 = 1.4400000...; the endlessly repeated sequence is the one-digit sequence "0".
A repeat unit is sometimes called a mer (or mer unit) in polymer chemistry. "Mer" originates from the Greek word meros, which means "a part". The word polymer derives its meaning from this, which means "many mers". A repeat unit (mer) is not to be confused with the term monomer, which refers to the small molecule from which a polymer is ...
(also written as 0. 9, 0.., or 0.(9)) is a repeating decimal that is an alternate way of writing the number 1. Following the standard rules for representing numbers in decimal notation, its value is the smallest number greater than or equal to every number in the sequence 0.9, 0.99, 0.999, .... It can be proved that this number is 1; that is,
The numbers 200-900 would be confused easily with 22 to 29 if they were used in chemistry. khīlioi = 1000, diskhīlioi = 2000, triskhīlioi = 3000, etc. 13 to 19 are formed by starting with the Greek word for the number of ones, followed by και (the Greek word for 'and'), followed by δέκα (the Greek word for 'ten').
For polymers in condensed chemical formulae, parentheses are placed around the repeating unit. For example, a hydrocarbon molecule that is described as CH 3 (CH 2) 50 CH 3, is a molecule with fifty repeating units. If the number of repeating units is unknown or variable, the letter n may be used to indicate this formula: CH 3 (CH 2) n CH 3.
Rules for calculating the periods of repeating decimals from rational fractions were given by James Whitbread Lee Glaisher in 1878. [5] For a prime p, the period of its reciprocal divides p − 1. [6] The sequence of recurrence periods of the reciprocal primes (sequence A002371 in the OEIS) appears in the 1973 Handbook of Integer Sequences.