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 tinynerdi April 3rd, 2010 07:23 PM

ring

compute the product in the given ring.
(-3,5)(2,-4) in Z_4 x Z_11

least common multiple is lcm(-3,5)=-15 and lsm(2,-4)=-4.

how do i find -15 in Z_4? and -4 in Z_11 if these was the right value.

 cknapp April 3rd, 2010 08:13 PM

Re: ring

Aren't products in the ring defined componentwise? I.e. (a,b)(c,d) = (ac,bd)

How does lcm come into play?

 tinynerdi April 4th, 2010 01:27 AM

Re: ring

(-3,5)(2,-4)= (-6,-20) = lcm (-6,-20)= -60? What make me confuse is the negative sign. Do we treat it the same as if it was positive?

 cknapp April 4th, 2010 04:09 PM

Re: ring

I'm still not sure what lcm has to do with it-- you just take the product componentwise, and reduce...

so (-6,-20). We need to reduce -6 (mod 4), and -20 (mod 11).
11|-22, and -20 = -22+2, so -20 = 2 (mod 11)
Likewise -6=-8+2, and 4|-8, so -6=2(mod 4),
so our answer is (2,2).

Does that make sense?

 tinynerdi April 4th, 2010 10:17 PM

Re: ring

yes thanks so much.

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