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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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    Perhaps [set membership.](http://en.wikipedia.org/wiki/Element_%28mathematics%29) Sea also Wikipedia's [list of math symbols.](http://en.wikipedia.org/wiki/List_of_mathematical_symbols) The notation dates back to Peano according to Jeff Miller's [Earliest Uses of Symbols of Set Theory and Logic:](http://jeff560.tripod.com/set.html) Giuseppe Peano (1858-1932) used an epsilon for membership in Arithmetices prinicipia nova methodo exposita, Turin 1889 (page vi, x). He stated that the symbol was an abbreviation for *est;* the entire work is in Latin.2012-11-10
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    Peano also used a backwards epsilon for "such that" in 1898, see [this prior question.](http://math.stackexchange.com/a/15460/242)2012-11-10
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    sideways u ... pointing which direction? Line through the middle: vertical, horizontal, diagonal?2012-11-10
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    @BillDubuque I know Peano came up with $\forall$, $\exists$, $\nexists$, etc. but I did not know he also invented $\ni$ for "such that".2012-11-10
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    No set membership is $\in$, not $\varepsilon$.2012-11-10
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    @ncmathsadist Surely $\in$ is one possible interpretation of the OP's description "looks like this: ε, but it's more like a sideways u with a line through the middle."2012-11-10
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    I am just looking at the symbol in front of me and giving the most plausible answer, the absence of any context.2012-11-10
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    Okay, cool down here...it's just a symbol :-)2012-11-10
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    @glebovg I am not sure where I learned that ∋ meant "such that", but I do remember learning that is is non-standard/uncommon, when I used it in an assignment and the grader had no idea what I was writing :)2012-11-19
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    Jessica asked the question, then it seems she never came back to see the answers.2012-11-19
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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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    Interestingly, I've seen $\ni$ where the an element of a set is to the right of the set, like $\supset$ is used for set membership.2012-11-10
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    You can use $a \in X$ or $X \ni a$, they mean the same (at least to all the books I've seen this in. But of course it's always better to just mention what you mean by a notation before using it.2012-11-10
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    @PatrickDaSilva I have not seen people use $\ni$ for membership. It makes more sense to use $\in$ because it looks like the letter e for element. It also looks like $\epsilon$, again epsilon (the first letter is e) for element.2012-11-10
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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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    Greeks have fonts, too.2012-11-10
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    @Jennifer It's called epsilon: $\epsilon$, which can be formatted on this site using `$\epsilon$`2012-11-10
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    Its variant is `\varepsilon`.2012-11-10