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Conversely the period of the repeating decimal of a fraction c / d will be (at most) the smallest number n such that 10 n − 1 is divisible by d. For example, the fraction 2 / 7 has d = 7, and the smallest k that makes 10 k − 1 divisible by 7 is k = 6, because 999999 = 7 × 142857. The period of the fraction 2 / 7 is ...
The extended Midy's theorem [2] states that if the repeating portion of the decimal expansion of a/p is divided into k-digit numbers, then their sum is a multiple of 10 k − 1. For example, 1 19 = 0. 052631578947368421 ¯ {\displaystyle {\frac {1}{19}}=0.{\overline {052631578947368421}}}
It was also used to mark Roman numerals whose values are multiplied by 1,000. [2] Today, however, the common usage of a vinculum to indicate the repetend of a repeating decimal [ 3 ] [ 4 ] is a significant exception and reflects the original usage.
For a repdigit to be prime, it must be a repunit (i.e. the repeating digit is 1) and have a prime number of digits in its base (except trivial single-digit numbers), since, for example, the repdigit 77777 is divisible by 7, in any base > 7.
For example, 0.24999... equals 0.25, exactly as in the special case considered. These numbers are exactly the decimal fractions, and they are dense. [41] [9] Second, a comparable theorem applies in each radix or base. For example, in base 2 (the binary numeral system) 0.111... equals 1, and in base 3 (the ternary numeral system) 0.222
Cyclic numbers are related to the recurring digital representations of unit fractions. A cyclic number of length L is the digital representation of 1/(L + 1). Conversely, if the digital period of 1/p (where p is prime) is p − 1, then the digits represent a cyclic number. For example: 1/7 = 0.142857 142857...
where the repeating block is indicated by dots over its first and last terms. [2] If the initial non-repeating block is not present – that is, if k = -1, a 0 = a m and = [;,, …, ¯], the regular continued fraction x is said to be purely periodic.
This produces a sequence of approximations, all of which are rational numbers, and these converge to the starting number as a limit. This is the (infinite) continued fraction representation of the number. Examples of continued fraction representations of irrational numbers are: √ 19 = [4;2,1,3,1,2,8,2,1,3,1,2,8,...] (sequence A010124 in the ...