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**VonNemo19** Hi. The proof that I'm working is in the title. The Defition of "same number as" is given as follows: $\displaystyle (1) $Two sets $\displaystyle S$ and $\displaystyle T$ are said to have the same number of elements if each element of $\displaystyle S$ can be paired with precisely one element of$\displaystyle T$ in such a way that every element of$\displaystyle T$ is paired with precisely one element of $\displaystyle S$.

Notation: If $\displaystyle \pi$ is a pairing of the elements of $\displaystyle S $with with those of Tand the element sof S is paired in $\displaystyle \pi$ to the element $\displaystyle t $of T$\displaystyle ,$ we shall write $\displaystyle s\overbrace{\leftrightarrow}^{\pi}{t}$ (I don't know how to put the \pi above the \leftrightarrow without the \overbrace).

I don't really understand the book's proof: Since $\displaystyle S$ has the same number of elements as $\displaystyle T$, we can select a pairing between the elements of $\displaystyle S $and $\displaystyle T$ in accordance with $\displaystyle (1). $We define a pairing as follows: If$\displaystyle s$ is paired with $\displaystyle t$ by the selected pairing $\displaystyle \pi$, then pair$\displaystyle t$ with $\displaystyle s$. That is if $\displaystyle s\overbrace{\leftrightarrow}^{\pi}{t}$, then$\displaystyle t$ is paired with $\displaystyle s$ to form the pairing of the elements $\displaystyle T$ with those of $\displaystyle S$. If the original pairing satisfied $\displaystyle (1), $then so will the new pairing. Specifically, since$\displaystyle \pi $had each element$\displaystyle T$ paired with a unique element of $\displaystyle S$, then the second pairing also has this property. Therefore, $\displaystyle T$ has the same number of elements as$\displaystyle S$. Please help me to understand why this proposition is not trivial, and also the procedure of the proof.