Tell me more ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.
up vote 4 down vote favorite
1

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

share|improve this question
2  
Perhaps 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. – Gone Nov 10 '12 at 1:46
1  
Peano also used a backwards epsilon for "such that" in 1898, see this prior question. – Gone Nov 10 '12 at 1:53
1  
sideways u ... pointing which direction? Line through the middle: vertical, horizontal, diagonal? – GEdgar Nov 10 '12 at 1:55
@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 1:56
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." – Gone Nov 10 '12 at 1:59
show 6 more comments

2 Answers

up vote 2 down vote

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".

share|improve this answer
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. – amWhy Nov 10 '12 at 1:52
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. – Patrick Da Silva Nov 10 '12 at 1:58
@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 1:59
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. – Patrick Da Silva Nov 10 '12 at 3:06
up vote 0 down vote

This is the Greek letter $\epsilon$, but the font is a little different like this $\varepsilon$.

share|improve this answer
1  
Greeks have fonts, too. – ncmathsadist Nov 10 '12 at 1:46
1  
@Jennifer It's called epsilon: $\epsilon$, which can be formatted on this site using $\epsilon$ – amWhy Nov 10 '12 at 1:55
Its variant is \varepsilon. – ncmathsadist Nov 10 '12 at 1:57

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.