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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2Perhaps set membership. Sea also Wikipedia's list of math symbols. The notation dates back to Peano according to Jeff Miller's Earliest Uses of Symbols of Set Theory and Logic: 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. – Bill Dubuque Nov 10 '12 at 01:46
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1Peano also used a backwards epsilon for "such that" in 1898, see this prior question. – Bill Dubuque Nov 10 '12 at 01:53
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1sideways u ... pointing which direction? Line through the middle: vertical, horizontal, diagonal? – GEdgar Nov 10 '12 at 01:55
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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". – glebovg Nov 10 '12 at 01:56
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No set membership is $\in$, not $\varepsilon$. – ncmathsadist Nov 10 '12 at 01:57
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2@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." – Bill Dubuque Nov 10 '12 at 01:59
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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. – ncmathsadist Nov 10 '12 at 02:00
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Okay, cool down here...it's just a symbol :-) – amWhy Nov 10 '12 at 02:00
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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 :) – Emily Nov 19 '12 at 21:05
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Jessica asked the question, then it seems she never came back to see the answers. – GEdgar Nov 19 '12 at 21:58
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@EdGorcenski I found this, where Prof. Farlow mentions $\ni $. I usually use $:$ to denote "such that". It makes more sense. – glebovg Nov 19 '12 at 22:30
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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".
glebovg
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2Interestingly, 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. – amWhy Nov 10 '12 at 01:52
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1You 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. – Patrick Da Silva Nov 10 '12 at 01:58
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1@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. – glebovg Nov 10 '12 at 01:59
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1Actually 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. – Patrick Da Silva Nov 10 '12 at 03:06
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This is the Greek letter $\epsilon$, but the font is a little different like this $\varepsilon$.
ncmathsadist
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1@Jennifer It's called epsilon: $\epsilon$, which can be formatted on this site using
$\epsilon$– amWhy Nov 10 '12 at 01:55 -