Freaking geometric product

[JonnyMaddox]

Hi, I just want to see if I understood this. Since the geometric product is associative and so on we can write for two multivectors A and B given by

$A= \alpha_{0}+\alpha_{1}e_{1}+\alpha_{2}e_{2}+\alpha_{3}e_{1}\wedge e_{2}$
$B= \beta_{0}+ \beta_{1}e_{1}+\beta_{2}e_{2}+\beta_{3}e_{1}\wedge e_{2}$

the geometric product multiplication as

$AB=M=\mu_{0}+\mu_{1}e_{1}+\mu_{2}e_{2}+\mu_{3}e_{1}e_{2}$

Where for example $\mu_{0}=\alpha_{0}\beta_{0}+\alpha_{1}\beta_{1}+\alpha_{2}\beta_{2}-\alpha_{3}\beta_{3}$ and so on.

Now let's take an example with beautiful vectors with numbers like $a = (e_{1}+2e_{2}), a_{1}=(2e_{1}+3e_{3}), a_{2}=(2e_{1}+0e_{2})$

So $aa_{1}= 8- e_1\wedge e_{2}$ Now what if I multiply this with $a_{2}$?

$(8-e_{1}\wedge e_{2})(2e_{1})$? Is it just $16e_{1}-(e_{1}\wedge e_{2})(2e_{1})$ ? My logic behind this is that one can symbolically (GP) multiply multivectors and then opens up his list with like 300 different products or something and then evaluates each of the products? Is this right?

 

[stevendaryl]

Hi, I just want to see if I understood this. Since the geometric product is associative and so on we can write for two multivectors A and B given by

$A= \alpha_{0}+\alpha_{1}e_{1}+\alpha_{2}e_{2}+\alpha_{3}e_{1}\wedge e_{2}$
$B= \beta_{0}+ \beta_{1}e_{1}+\beta_{2}e_{2}+\beta_{3}e_{1}\wedge e_{2}$

the geometric product multiplication as

$AB=M=\mu_{0}+\mu_{1}e_{1}+\mu_{2}e_{2}+\mu_{3}e_{1}e_{2}$

Where for example $\mu_{0}=\alpha_{0}\beta_{0}+\alpha_{1}\beta_{1}+\alpha_{2}\beta_{2}-\alpha_{3}\beta_{3}$ and so on.

Now let's take an example with beautiful vectors with numbers like $a = (e_{1}+2e_{2}), a_{1}=(2e_{1}+3e_{3}), a_{2}=(2e_{1}+0e_{2})$

So $aa_{1}= 8- e_1\wedge e_{2}$ Now what if I multiply this with $a_{2}$?

$(8-e_{1}\wedge e_{2})(2e_{1})$? Is it just $16e_{1}-(e_{1}\wedge e_{2})(2e_{1})$ ? My logic behind this is that one can symbolically (GP) multiply multivectors and then opens up his list with like 300 different products or something and then evaluates each of the products? Is this right?

To me, if you're dealing with geometric algebra, it's better to leave out the $\wedge$. For vectors, $A \wedge B = \frac{1}{2}(AB - BA)$. If $A$ and $B$ are orthogonal, then $A \wedge B = AB$

So $aa_{1}= 8- e_1 e_{2}$.
Then when we multiply by $2 e_1$ you just get:

$8 e_1 - e_1 e_2 e_1$

You can simplify using $e_2 e_1 = -e_1 e_2$ to get:
$8 e_1 + e_1 e_1 e_2$

Then you can use the fact that $e_1 e_1 = 1$ to get:
$8 e_1 + e_2$

(Note: this is assuming that your basis vectors are orthonormal:
$e_i e_j + e_j e_i = 2 \delta_{ij}$)

 

[JonnyMaddox]

Hi stevendaryl,
I think I got it. If we can reformulate everything as GP then we can always use this simple relations between the orthogonal basis vectors, hm that was more then obvious...ok thank you.

When you multiply $8-e_{1}e_{2}$ by $2e_{1}$ where did you leave the factor of $2$ in the result? I think it should be a rotation by 90 degrees anti-clockwise and dilation by factor 2?

Thx for your reply !

 

[stevendaryl]

Hi stevendaryl,
I think I got it. If we can reformulate everything as GP then we can always use this simple relations between the orthogonal basis vectors, hm that was more then obvious...ok thank you.

When you multiply $8-e_{1}e_{2}$ by $2e_{1}$ where did you leave the factor of $2$ in the result? I think it should be a rotation by 90 degrees anti-clockwise and dilation by factor 2?

Thx for your reply !

That was a typo, sorry.

 

[blue_raver22]

Yes, you are correct in your understanding of the geometric product and its properties. The geometric product allows for the multiplication of multivectors, which are mathematical objects that extend the concept of vectors in traditional vector algebra. This product is associative, meaning that the order of multiplication does not matter, and it follows certain rules and properties that make it useful in various applications.

In your example, you correctly applied the geometric product to multiply the multivectors aa_{1} and a_{2}. This results in a scalar component of 8 and a bivector component of -e_{1}\wedge e_{2}. When multiplied by a_{2}, the result is a new multivector with a scalar component of 16e_{1} and a bivector component of -(e_{1}\wedge e_{2})(2e_{1}), as you stated.

Your understanding of using the geometric product to evaluate multiple products is also correct. One can symbolically multiply multivectors and then evaluate each product using the properties of the geometric product. This can be helpful in simplifying complex expressions and solving problems in various fields such as physics, engineering, and computer graphics.

Overall, your understanding of the geometric product and its application is accurate. Keep exploring and practicing with this concept to deepen your understanding and its potential uses in different areas of science and mathematics.