# define x ~ y

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• Nov 30th 2011, 10:57 AM
nep977
define x ~ y
If G is a group with subgroups A and B, then how do you define "x ~y" if and only if there exists a in A and b in B such that x = ayb?
• Nov 30th 2011, 12:06 PM
Deveno
Re: define x ~ y
that IS the definition.

i suspect what you want to do is show "~" is an equivalence relation; that is, show ~ is reflexive, symmetric and transitive.
• Nov 30th 2011, 12:31 PM
nep977
Re: define x ~ y
ok so
R is an equivalence relation

Its reflexive: y ~ y
Symmetry: x ~ y then y ~ x

is it like this? i don't get the part "if and only if there exists a in A and b in B such that x = ayb"
what do i do with x = ayb
• Nov 30th 2011, 12:39 PM
Deveno
Re: define x ~ y
to prove that ~ is reflexive, you need to prove that for ANY x in G, you can find a in A and b in B such that x = axb.

you can't just say that x~x, because you don't KNOW that, unless you can actually produce the elements a and b.
• Nov 30th 2011, 01:10 PM
nep977
Re: define x ~ y
ok got it. Thank you