Mapping/Examples/x^3 + y^4 = 1

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Example of Relations which is not a Mapping

Let $R_4$ be the relation defined on the Cartesian plane $\R \times \R$ as:

$R_4 = \set {\tuple {x, y} \in \R \times \R: x^3 + y^4 = 1}$

Then $R_4$ is not a mapping.


Proof

Graph of $x^3 + y^4 = 1$

$R_4$ fails to be a mapping for the following reasons:


$(1): \quad$ For $x > 1$, there exists no $y \in \R$ such that $x^3 + y^4 = 1$.

Thus $R_4$ fails to be left-total.


$(2): \quad$ For $x < 1$, there exist exactly two $y \in \R$ such that $x^3 + y^4 = 1$, for example:

\(\displaystyle 0^3 + 1^4\) \(=\) \(\displaystyle 1\)
\(\displaystyle 0^3 + \paren {-1}^4\) \(=\) \(\displaystyle 1\)

So both $\tuple {0, 1}$ and $\tuple {0, -1}$ are elements of $R_4$.

Thus $R_4$ fails to be many-to-one.

$\blacksquare$


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