5
$\begingroup$

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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