For any integer coprime to 10, its reciprocal is a repeating decimal without any non-recurring digits. E.g. 1⁄143 = 0.006993006993006993...
While the expression of a single series with vinculum on top is adequate, the intention of the above expression is to show that the six cyclic permutations of 006993 can be obtained from this repeating decimal if we select six consecutive digits from the repeating decimal starting from different digits.
This illustrates that cyclic permutations are somehow related to repeating decimals and the corresponding fractions.
The greatest common divisor (gcd) between any cyclic permutation of an m-digit integer and 10m − 1 is constant. Expressed as a formula,
where N is an m-digit integer; and Nc is any cyclic permutation of N.
For example,
If N is an m-digit integer, the number Nc, obtained by shifting N to the left cyclically, can be obtained from:
where d is the first digit of N and m is the number of digits.
This explains the above common gcd and the phenomenon is true in any base if 10 is replaced by b, the base.
The cyclic permutations are thus related to repeating decimals, the corresponding fractions, and divisors of 10m−1. For examples the related fractions to the above cyclic permutations are thus:
Reduced to their lowest terms using the common gcd, they are:
That is, these fractions when expressed in lowest terms, have the same denominator. This is true for cyclic permutations of any integer.
An integral multiplier refers to the multiplier n being an integer:
It is necessary for F to be coprime to 10 in order that 1⁄F is a repeating decimal without any preceding non-repeating digits (see multiple sections of Repeating decimal). If there are digits not in a period, then there is no corresponding solution.
For these two cases, multiples of X, i.e. (j X) are also solutions provided that the integer i satisfies the condition n j⁄F < 1. Most often it is convenient to choose the smallest F that fits the above. The solutions can be expressed by the formula:
To exclude integers that begin with zeros from the solutions, select an integer j such that j⁄F > 1⁄10, i.e. j > F⁄10.
There is no solution when n > F.
An integer X shift left cyclically by k positions when it is multiplied by a fraction n⁄s. X is then the repeating digits of s⁄F, whereby F is F0 = s 10k - n, or a factor of F0; and F must be coprime to 10.
For this third case, multiples of X, i.e. (j X) are again solutions but the condition to be satisfied for integer j is that n j⁄F < 1. Again it is convenient to choose the smallest F that fits the above.
The solutions can be expressed by the formula:
To exclude integers that begin with zeros from the solutions, select an integer j such that j s⁄F > 1⁄10, i.e. j > F⁄10s.
Again if j s⁄F > 1, there is no solution.
The direct algebra approach to the above cases integral multiplier lead to the following formula:
A long division of 1 by 7 gives:
At the last step, 1 reappears as the remainder. The cyclic remainders are {1, 3, 2, 6, 4, 5}. We rewrite the quotients with the corresponding dividend/remainders above them at all the steps:
and also note that:
By observing the remainders at each step, we can thus perform a desired cyclic permutation by multiplication. E.g.,
In this manner, cyclical left or right shift of any number of positions can be performed.
Less importantly, this technique can be applied to any integer to shift cyclically right or left by any given number of places for the following reason:
An integer X shift cyclically right by k positions when it is multiplied by an integer n. Prove its formula.
Proof
First recognize that X is the repeating digits of a repeating decimal, which always possesses cyclic behavior in multiplication. The integer X and its multiple n X then will have the following relationship:
This completes the proof.
An integer X shift cyclically left by k positions when it is multiplied by an integer n. Prove its formula.
First recognize that X is the repeating digits of a repeating decimal, which always possesses a cyclic behavior in multiplication. The integer X and its multiple n X then will have the following relationship:
which represents the results after left cyclical shift of k positions.
This completes the proof. The proof for non-integral multiplier such as n⁄s can be derived in a similar way and is not documented here.
The permutations can be:
Main article: parasitic number
When a parasitic number is multiplied by n, not only it exhibits the cyclic behavior but the permutation is such that the last digit of the parasitic number now becomes the first digit of the multiple. For example, 102564 x 4 = 410256. Note that 102564 is the repeating digits of 4⁄39 and 410256 the repeating digits of 16⁄39.
An integer X shift right cyclically by double positions when it is multiplied by an integer n. X is then the repeating digits of 1⁄F, whereby F = n × 102 - 1; or a factor of it; but excluding values for which 1⁄F has a period length dividing 2 (or, equivalently, less than 3); and F must be coprime to 10.
Most often it is convenient to choose the smallest F that fits the above.
The following multiplication moves the last two digits of each original integer to the first two digits and shift every other digits to the right:
period = 99i.e. 99 repeating digits.
period = 66
299 = 13×23
some special cases are illustrated below
period = 6
period = 22
period = 18
399 = 3×7×19
499 is a full reptend prime
Note that:
There are many other possibilities.
Problem: An integer X shift left cyclically by single position when it is multiplied by 3. Find X.
Solution: First recognize that X is the repeating digits of a repeating decimal, which always possesses some interesting cyclic behavior in multiplications. The integer X and its multiple then will have the following relationship:
This yields the results that:
The other solution is represented by 2⁄7 x 3 = 6⁄7:
There are no other solutions 1 because:
However, if the multiplier is not restricted to be an integer (though ugly), there are many other solutions from this method. E.g., if an integer X shift right cyclically by single position when it is multiplied by 3⁄2, then 3 shall be the next remainder after 2 in a long division of a fraction 2⁄F. This deduces that F = 2 x 10 - 3 = 17, giving X as the repeating digits of 2⁄17, i.e. 1176470588235294, and its multiple is 1764705882352941.
The following summarizes some of the results found in this manner:
A 2-parasitic number
4⁄19, 6⁄19, 8⁄19, 10⁄19, 12⁄19, 14⁄19, 16⁄19, 18⁄19
An integer X shift left cyclically by double positions when it is multiplied by an integer n. X is then the repeating digits of 1⁄F, whereby F is R = 102 - n, or a factor of R; excluding values of F for which 1⁄F has a period length dividing 2 (or, equivalently, less than 3); and F must be coprime to 10.
The following summarizes some of the results obtained in this manner, where the white spaces between the digits divide the digits into 10-digit groups:
1⁄93, 2⁄93, 4⁄93, 5⁄93, 7⁄93, 8⁄93, 10⁄93, 11⁄93, 13⁄93
1⁄87, 2⁄87, 4⁄87, 5⁄87, 6⁄87
In duodecimal system, the transposable integers are: (using inverted two and three for ten and eleven, respectively)
Note that the “Shifting left cyclically by single position” problem has no solution for the multiplier less than 12 except 2 and 5, the same problem in decimal system has no solution for the multiplier less than 10 except 3.
P. Yiu, k-right-transposable integers, Chap.18.1 'Recreational Mathematics' ↩