# Complete the group table

• Jan 19th 2010, 08:11 PM
ChrisBickle
Complete the group table
Ok the question says complete the group table to G = {a,b,c,d}
We are given this

x a b c d
a
b a
c a
d

I know that each element can occur only once per row/column and that one element needs to be the identity i just cant figure out which one im pretty sure if you figure out what element is the identity it wouldnt be to hard to fill in the rest im just unsure about how to figure it out.
• Jan 19th 2010, 08:12 PM
Drexel28
Quote:

Originally Posted by ChrisBickle
Ok the question says complete the group table to G = {a,b,c,d}
We are given this

x a b c d
a
b a
c a
d

I know that each element can occur only once per row/column and that one element needs to be the identity i just cant figure out which one im pretty sure if you figure out what element is the identity it wouldnt be to hard to fill in the rest im just unsure about how to figure it out.

have you filled in anything else?
• Jan 19th 2010, 08:23 PM
ChrisBickle
actually i just looked at it that its not right what i posted i should have b * b = a and a * c = a other than that im not sure where to go there is nothing (i dont think giving where the other a should go. I guess a * c = a so there is a possibility c is the identity?
• Jan 19th 2010, 08:26 PM
Drexel28
Quote:

Originally Posted by ChrisBickle
actually i just looked at it that its not right what i posted i should have b * b = a and a * c = a other than that im not sure where to go there is nothing (i dont think giving where the other a should go. I guess a * c = a so there is a possibility c is the identity?

$ac=a\implies c=aa^{-1}=e$. So yes, $c=e$. From there you should be able to fill in quite a few squares.
• Jan 20th 2010, 02:10 AM
tonio
Quote:

Originally Posted by ChrisBickle
Ok the question says complete the group table to G = {a,b,c,d}
We are given this

x a b c d
a
b a
c a
d

I know that each element can occur only once per row/column and that one element needs to be the identity i just cant figure out which one im pretty sure if you figure out what element is the identity it wouldnt be to hard to fill in the rest im just unsure about how to figure it out.

If I understand correctly, you have $a$ twice in the first column and this cannot be in a group's multiplication table...

Tonio
• Jan 20th 2010, 02:16 AM
Swlabr
Quote:

Originally Posted by tonio
If I understand correctly, you have $a$ twice in the first column and this cannot be in a group's multiplication table...

Tonio

He corrected himself. I believe the table should should start off as,

x a b c d
a
b ..a
c a
d

This means that c is the identity, and so we have an element which isn't of order 2 in a group of order 4, namely b.

Anyway, the solution is got from here by filling in what the identity does, and then by pretending it is a simplified suduko...