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Why is closure omitted in some group definitions?

I have just started studying about groups, so pardon me if this is a lame question

while i was reading the definition. the author states that

'strictly speaking the closure law is not necessary since a binary operation on a set necessarily satisfies it'

can anyone elaborate on this statement?i am not sure i quite understand it

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marked as duplicate by lhf, Zev Chonoles Oct 30 '11 at 15:38

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1 Answer 1

up vote 3 down vote accepted

He just means that the definition of a binary operation includes closure, so it is not strictly necessary to include it again in the definition of a group.

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