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$\begingroup$

The symbol kind of looks like this: ε, but it's more like a sideways u with a line through the middle.

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    @EdGorcenski I found [this](http://math.umaine.edu/~farlow/sec13.pdf), where Prof. Farlow mentions $\ni $. I usually use $:$ to denote "such that". It makes more sense.2012-11-19

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Do you mean $\in$? This means "an element of". For example, if we denote the set of natural numbers by ${\mathbb N}$ then $1 \in {\mathbb N}$. Similarly, $1,2,3, \ldots \in {\mathbb N}$, and $ - 1 \notin {\mathbb N}$. Sometimes you might also see $\ni$, which some authors use for "such that". You might also be referring to $\epsilon$, which is the same as $\varepsilon $, or perhaps you mean $\not\subset$, which usually means "not a subset of".

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    Actually I never see people using $\epsilon$ to denote membership. I know that the symbol $\in$ is essentially an epsilon but I reserve $\varepsilon$ for numbers and $\in$ for membership, this thing $\epsilon$, I hate it.2012-11-10
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This is the Greek letter $\epsilon$, but the font is a little different like this $\varepsilon$.

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    Its variant is `\varepsilon`.2012-11-10