Can someone please advise as to the meaning of the symbol here?
Please note that this is a symbol utilized in logic or possible calculations utilizing triangles (though still logical).
Its not the euro...
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It's not a "c", it's an epsilon. Since Wikipedia says "The lunate epsilon (ϵ) is not to be confused with the set membership symbol (∈)", it might be the case that the set membership symbol doesn't even derive from the lunate epsilon. However, since I've heard logicians refer to the set membership symbol as "the epsilon relation" (as in "ordinals are well-ordered by the epsilon relation), I'd guess that it does derive from the epsilon. – Quinn Culver Jun 13 '12 at 18:22
The symbol
∈
denotes membership in a set.
For example, if we say that
A ∈ Δ
then that means the constant A is a member of the set Δ.
The opposite would be
∉
which denotes that something is not a member of a set.
For example, if we say that
X ∉ Δ
then that means the constant X is not a member of the set Δ.
Also see the Wikipedia article on elements (in mathematics).
