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\title{\bf Announcement 237: A reality of the division by zero $z/0=0$ by geometrical optics}
\author{{\it Institute of Reproducing Kernels}\\

\date{\today}
\maketitle
{\bf Abstract: } In this announcement, we shall state a reality of the division by zero $z/0=0$ by the reflection (geometrical optics) and from this fact we will be able to understand that the division by zero $z/0=0$ is natural in both mathematics and our physical world.
\bigskip
\section{Introduction}
%\label{sect1}
By {\bf a natural extension of the fractions}
\begin{equation}
\frac{b}{a}
\end{equation}
for any complex numbers $a$ and $b$, we, recently, found the surprising result, for any complex number $b$
\begin{equation}
\frac{b}{0}=0,
\end{equation}
incidentally in \cite{s} by the Tikhonov regularization for the Hadamard product inversions for matrices, and we discussed their properties and gave several physical interpretations on the general fractions in \cite{kmsy} for the case of real numbers. The result is a very special case for general fractional functions in \cite{cs}.　
The division by zero has a long and mysterious story over the world (see, for example, google site with division by zero) with its physical viewpoints since the document of zero in India on AD 628, however,
Sin-Ei, Takahasi (\cite{taka}) (see also \cite{kmsy}) established a simple and decisive interpretation (1.2) by analyzing some full extensions of fractions and by showing the complete characterization for the property (1.2). His result will show that {\bf our mathematics says} that the result (1.2) should be accepted as a natural one:
\bigskip
{\bf Proposition. }{\it Let F be a function from ${\bf C }\times {\bf C }$ to ${\bf C }$ such that
$$
F (b, a)F (c, d)= F (bc, ad)
$$
for all
$$
a, b, c, d \in {\bf C }
$$
and
$$
F (b, a) = \frac {b}{a }, \quad a, b \in {\bf C }, a \ne 0.
$$
Then, we obtain, for any $b \in {\bf C } $
$$
F (b, 0) = 0.
$$
}
\medskip
Furthermore, note that Hiroshi Michiwaki with his 6 year old daughter gave the important interpretation of the division by zero $z/0=0$ by the intuitive meaning of the division, {\bf independently of the concept of the product }(see \cite{ann}) . See \cite{ann} for the basic meanings of the division by zero.
We shall state a reality of the division by zero $z/0=0$ by the concept of reflection (geometrical optics). It seems that the common interpretations for the reflections for the center of a circle and the point at infinity are not suitable.
\section{Reflection points}
For simplicity, we shall consider the unit circle ${|z| = 1}$ on the complex $z = x +iy$ plane.
Then, we have the reflection formula
\begin{equation}
z^* = \frac{1}{\overline{z}}
\end{equation}
for any point $z$, as well-known (\cite{ahlfors}). For the reflection point $z^*$, there is no problem for the points
$z \neq 0, \infty$. 　As the classical result, the reflection of zero is the point at infinity and conversely, for the point at infinity we have the zero point. The reflection is a one to one and onto mapping between the inside and the outside of the unit circle.
However, we wonder the following common facts:
Are these correspondences suitable?
Does there exist the point at $\infty$, really?
Is the point at infinity corresponding to the zero point? Is the point at $\infty$ reasonable from the practical point of view?
Indeed, where can we find the point at infinity? Of course, we know plesantly the point at infinity
on the Riemann sphere, however on the complex $z$-plane it seems that we can not find the corresponding point. When we approach to the origin on a radial line, it seems that the correspondence reflection points approach to {\it the point at infinity} with the direction (on the radial line).
\section{Interpretation by the division by zero $z/0=0$}
On the concept of the division by zero, there is no the point at infinity $\infty$ as the numbers. For any point $z$ such that $|z| >1$, there exists the unique point $z^*$ by (2.1). Meanwhile, for any point $z$ such that $|z| < 1$ except $z=0$, there exits the unique point $z^*$ by (2.1).
Here, note that for $z=0$, by the division by zero, $z^*=0$. Furthermore, we can see that
\begin{equation}
\lim_{z \to 0}z^* =\infty,
\end{equation}
however, for $z=0$ itself, by the division by zero, we have $z^*=0$. This will mean a strong discontinuity of the function
\begin{equation}
W = \frac{1}{z}
\end{equation}
at the origin $z=0$; that is a typical property of the division by zero. This strong discontinuity may be looked in the above reflection property, physically.
\section{Conclusion}
{\Large \bf Should we exclude the point at infinity, from the numbers?} We were able to look the strong discontinuity of the division by zero in the reflection with respect to circles, physically ( geometrical optics ).
The division by zero gives a one to one and onto mapping of the reflection (2.1) from the whole complex plane onto the whole complex plane.
{\Large \bf The infinity $\infty$ may be considered as in (3.1) as the usual sense of limits,} however, the infinity $\infty$ is not a definite number.
\bigskip
\bibliographystyle{plain}
\begin{thebibliography}{10}
\bibitem{ahlfors}
L. V. Ahlfors, Complex Analysis, McGraw-Hill Book Company, 1966.
\bibitem{cs}
L. P. Castro and S.Saitoh, Fractional functions and their representations, Complex Anal. Oper. Theory {\bf7} (2013), no. 4, 1049-1063.
\bibitem{kmsy}
S. Koshiba, H. Michiwaki, S. Saitoh and M. Yamane,
An interpretation of the division by zero z/0=0 without the concept of product
(note).
\bibitem{kmsy}
M. Kuroda, H. Michiwaki, S. Saitoh, and M. Yamane,
New meanings of the division by zero and interpretations on $100/0=0$ and on $0/0=0$,
Int. J. Appl. Math. Vol. 27, No 2 (2014), pp. 191-198, DOI: 10.12732/ijam.v27i2.9.
\bibitem{mst}
H. Michiwaki, S. Saitoh, and M. Takagi,
A new concept for the point at infinity and the division by zero z/0=0
(note).
\bibitem{s}
S. Saitoh, Generalized inversions of Hadamard and tensor products for matrices, Advances in Linear Algebra \& Matrix Theory. Vol.4 No.2 (2014), 87-95. http://www.scirp.org/journal/ALAMT/
\bibitem{taka}
S.-E. Takahasi,
{On the identities $100/0=0$ and $ 0/0=0$}
(note).
\bibitem{ttk}
S.-E. Takahasi, M. Tsukada and Y. Kobayashi, Classification of continuous fractional binary operators on the real and complex fields, Tokyo Journal of Mathematics (in press).
\bibitem{ann}
Announcement 185: Division by zero is clear as z/0=0 and it is fundamental in mathematics,
Institute of Reproducing Kernels, 2014.10.22.
\end{thebibliography}
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