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$\begingroup$

Anybody who knows/can explain what this character means?

enter image description here

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    It tells you it's the symmetric set difference. – 2017-02-26
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    Well it's said in the text – 2017-02-26
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    And it is encoded as `\oplus`. – 2017-02-26

2 Answers 2

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enter image description hereThe symbol is symmetric set difference,
$A \oplus B = (A-B) \cup (B-A)$.
The red colour portion shows the symmetric difference.

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    What does $\ominus$ and $\otimes$ mean ? – 2017-02-26
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This is symmetric difference:

$$A \oplus B:=(A-B) \cup (B-A).$$

It is the collection of elements in either of the sets, but not the intersection. Associativity is probably easiest using:

$$(A-B) \cup (B-A)=(A \cap B^c)\cup (A^c \cap B),$$

Where $A^c$ denotes the set theoretic complement.