# How to find if neutral/idnty element exists for 16 different binary operations of *?

• Aug 24th 2012, 04:49 PM
x3bnm
How to find if neutral/idnty element exists for 16 different binary operations of *?
Suppose I'm given an operation $\displaystyle *$ on a set $\displaystyle A\,\, \text{where}\,\, a,b \in A$ such that the operation between two elements of set $\displaystyle A$ is also an element of set $\displaystyle A$

Now there are $\displaystyle 2 \times 2 \times 2 \times 2 = 16$ possible binary operations of $\displaystyle *$ on set $\displaystyle A$.

If I'm asked to find if there exists a neutral/identity element of a certain table where $\displaystyle a * a, a * b, b * a \text{ and } b * b$
are defined, how do I figure that out?

What I mean is suppose 1 of 16 table of operations looks like this:

$\displaystyle \begin{array}{|l||r|} \hline (a * a) & a \\ \hline (a * b) & b \\ \hline (b * a) & a \\ \hline (b * b) & b \\ \hline \end{array}$

How do I figure out if an identity element exist for this table? Also is it possible to find the identity/neutral element from this table?
• Aug 24th 2012, 05:48 PM
HallsofIvy
Re: How to find if neutral/idnty element exists for 16 different binary operations of
That binary operation does not have an identity. The definition of identity is "e is an identity for operation "*" if and only if e*x= x*e= x for every x in in the set". Here, a*b= b but $\displaystyle b*a= a\ne b$ so a is NOT an identity. Similarly, b*a= a but $\displaystyle a*b= b\ne a$ so b is not an identity.
• Aug 24th 2012, 07:15 PM
x3bnm
Re: How to find if neutral/idnty element exists for 16 different binary operations of
Again thanks.