# Questions tagged [definition]

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### When were equivalence classes formalized?

Neither wikipedia or the first few pages of Google are showing me much about the history of the development of equivalence classes. When was this notion first formalized? Footnote: I originally asked ...
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### What did Euclid mean by a straight line in his time?

The third and fourth definitions in Euclid's Elements say: The ends of a line are points. A straight line is a line which lies evenly with the points on itself. The fourth definition is usually ...
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### Definition of ordinal multiplication

The ordinal multiplication $\cdot$ can be defined recursively via ordinal addition $+$ for any ordinal $\alpha$ as follows: $\alpha\cdot 0=0$. $\alpha\cdot (\beta+1)=\alpha\cdot \beta+\alpha$ for any ...