# Math Help - Check if ive done right please binary operations

1. ## Check if ive done right please binary operations

An operation * is defined on a set of numbers S by x * y = x + y - 2(x^2)(y^2)

in z+, q and q+

for z + its not a binary operation because you can get a negative answer say x = 3 and y = 4

for q it is a binary operation

for q+ it is a binary operation

An operation * is defined on a set of numbers S by x * y = x + y - 2(x^2)(y^2)

in z+, q and q+

for z + its not a binary operation because you can get a negative answer say x = 3 and y = 4

for q it is a binary operation

for q+ it is a binary operation
You are correct to note that $*:S^2\mapsto S$ so when $S=\mathbb{N}$ we ave that $*(3,4)\notin\mathbb{N}$. But, why doesn't that exact same example work when $S=\mathbb{Q}^+$?

An operation $*$ is defined on a set of numbers $S$ by $x * y = x + y - 2x^2y^2$

in $\mathbb{Z}^+$, $\mathbb{Q}$ and $\mathbb{Q}^+$

for $\mathbb{Z}^+$ its not a binary operation because you can get a negative answer say $x = 3$ and $y = 4$

for $\mathbb{Q}$ it is a binary operation

for $\mathbb{Q}^+$ it is a binary operation
Since $(3,4)\in\mathbb{Q}^+\times\mathbb{Q}^+$, then clearly $\mathbb{Q}^+\times\mathbb{Q}^+$ is not a valid domain for the above function into $\mathbb{Q}^+$.

However, $\mathbb{Q}\times\mathbb{Q}$ is a valid domain into $\mathbb{Q}$.

4. Originally Posted by hatsoff
Since $(3,4)\in\mathbb{Q}^+\times\mathbb{Q}^+$, then clearly $\mathbb{Q}^+\times\mathbb{Q}^+$ is not a valid domain for the above function into $\mathbb{Q}^+$.

However, $\mathbb{Q}\times\mathbb{Q}$ is a valid domain into $\mathbb{Q}$.
I'm sorry. Did I say something incorrect?

5. Originally Posted by Drexel28
I'm sorry. Did I say something incorrect?
Oh no. It just takes me longer to type than you, apparently, so that I logged my response after yours even though we set out at approximately the same time.

6. Sorry i dont understand why q+ is not a binary operation. I thought q meant fractions. Why are you using 3 and 4?

Sorry i dont understand why q+ is not a binary operation. I thought q meant fractions. Why are you using 3 and 4?
3 and 4 are fractions:

$3 = \frac{3}{1}$,

$4 = \frac{4}{1}$.

8. Originally Posted by Swlabr
3 and 4 are fractions:

$3 = \frac{3}{1}$,

$4 = \frac{4}{1}$.
woops