Friday 16 June 2017

self learning - ab = a/b = b/a, what's this symmetry called?

I was playing around with numbers the other day, and I found an interesting symmetry, that I would like to know if it has any specific name assigned to it.




Let's assume the notation $n:a$ to refer to any number whose modulus $n = a$. That is, which if written with an $n$-radixed numeral system, would have a least significant digit of $a$. (If anyone knows the standard notation for expressing this, feel free to add that info.)



We can then write $10:3 \times 10:7 = 10:1$, because if we multiply any two integers with least significant decimals of $3$ and $7$ respectively, then the result will always have a least significant decimal of $1$, for example:



$$\begin{align}
33 \times 77 &= 2541\\
103 \times 97 &= 9991\\
73 \times 37 &= 2701
\end{align}$$




We can also write $\dfrac{10:3}{10:7} = 10:9$, because if we divide a number with least significant decimal of $3$, by a number with least significant decimal of $7$, then (given that it is evenly divisible), the result will always have a last decimal of $9$. For example:



$$\begin{align}
\frac{7743}{87} &= 89\\
\frac{1083}{57} &= 19\\
\frac{3353}{7} &= 479
\end{align}$$



So far, nothing weird is going on. However, the strange thing starts to happen if we use certain radixes, for example 24 instead of 10, then




$$(24:17 \times 24:23) = \left(\frac{24:17}{24:23}\right) = \left(\frac{24:23}{24:17}\right) = 24:7$$



Let us simplify the above a bit, and write it as:



$$24:\left((17 \times 23) = \frac{17}{23} = \frac{23}{17} = 7\right)$$



Let us try to test the truthness of this. By creating the numbers $24:17$ and $24:23$, and multiplying them, we can test if the product $= 24:7$.
If $24:\left(\frac{23}{17} = 7\right)$, then $24:((17 \times 7) = 23)$, and if $24:(\frac{17}{23} = 7)$, then $24:((7 \times 23) = 17)$. In order to test the above expression, let us therefore multiply the numbers:





  1. $24:17$ and $24:23$ (the result should be $24:7$)

  2. $24:23$ and $24:7$ (the result should be $24:17$)

  3. $24:17$ and $24:7$ (the result should be $24:23$)



For this demonstration I will only do the multiplication once for each of the three types, above, but of course one can try as many combinations of numbers with $24$-moduluses specified above that one feels is sufficient to convince oneself that it is always true.



A number that equals $24:17$, that is, which has a $24$-modulus of $17$, must have the form $17 + 24 \times x$, where $x$ is any arbitrary integer. We can use the same method to create arbitrary numbers with any specific modulus. For this test, let's let $x = 0$. Let us use these arbitrary numbers with moduluses $17$, $7$ and $23$:



$$\begin{align}

17 + 0 \times 24 &= 17\\
7 + 0 \times 24 &= 7\\
23 + 0 \times 24 &= 23
\end{align}$$



Then let us examine if there is a symmetry between the result of them being multiplied and divided by one another.



$$\begin{align}
17 \times 7 &= 119\\
\frac{119}{24} &= 4 + \frac{23}{24}

\end{align}$$



The above shows that $24:\left(\frac{23}{17} = 7\right)$



$$\begin{align}
23 \times 7 &= 161\\
\frac{161}{24} &= 6 + \frac{17}{24}
\end{align}$$



The above shows that $24:\left(\frac{17}{23} = 7\right)$




$$\begin{align}
17 \times 23 = 391\\
\frac{391}{24} = 16 + \frac7{24}
\end{align}$$



The above shows that $24:((17 \times 23) = 7)$

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