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Is the set of even integers closed under addition and multiplication?
Yes.
Is a set closed with respect to multiplication is it also close with respect to addition?
No. An addition operation need not even be defined.
Are even numbers closed in addition?
Yes, they are.
Is there a subset of the natural numbers that is closed for addition?
Yes. The entire set of natural numbers is closed under addition (but not subtraction). So are the even numbers (but not the odd numbers), the multiples of 3, of 4, etc.
What is index in group theory?
Say we have a group G, and some subgroup H. The number of cosets of H in G is called the index of H in G. This is written [G:H].If G and H are finite, [G:H] is just |G|/|H|.What if they are infinite? Here is an example. Let G be the integers under addition. Let H be the even integers under addition, a subgroup. The cosets of H in G are H and H+1. H+1 is the set of all even integers + 1, so the set of all odd integers. Here we have partitioned the integers into two cosets, even and odd integers. So [G:H] is 2.
