Lisp Patterns Question W.R.T. Clifford Algebras May 20th, 2009
Patrick Stein

Since I moved my website to WordPress, I have been watching what search-engine searches land people on my page.

Sadly, two of the things that get people to my page most often still have not been moved over to the new site from the old. One of those things is a C++ template library for Clifford algebras. I am going to move that stuff over this week, I promise. But, I thought I might also make a Lisp library for Clifford algebras, too.

An example Clifford algebra might be C_{3,0}. A generic element in this algebra is of the form:

a_0 + a_1 e_1 + a_2 e_2 + a_3 e_3 + a_{1,2} e_{1,2} + a_{1,3} e_{1,3} + a_{2,3} e_{2,3} + a_{1,2,3} e_{1,2,3}

where the a_* are coefficients. For ease in dealing with these items, I will probably store that in a vector that looks something like this:
#(a_0 #(a_1 a_2 a_3) #(a_{1,2} a_{1,3} a_{2,3}) a_{1,2,3})

If you want a simple element though (where most of the coefficients are zero), you shouldn’t have to stare down all of that stuff or remember that a_{1,3} comes before a_{2,3}. I want you to be able to make 3 + 4 e_1 + 5 e_{1,2,3} more simply. Something like one of the following:

(defvar *a* (ca-element '(3 (4 1) (5 1 2 3)) :p 3))
(defvar *a* (ca-element '(3 (4 . :e1) (5 . :e1-2-3)) :p 3))
(defvar *a* (ca-element :s 3 :e1 4 :e1-2-3 5 :p 3))

Similarly, I will define something akin to (aref …) so that one can check or change a_{1,2} like one of the following:

(caref *a* 1 2)
(caref *a* :e1-2)

By analogy with the complex numbers, I should instead have:

(e1-2 *a*)

But, that just doesn’t scale very well.

I am leaning toward using a list of numbers to tell which a_* is being referenced. This would allow greater flexibility in other functions. But, it’s also less mathy.

Does anyone have any suggestions? Should I really be supporting both? Through the same function names or through (caref …) and (caref* …) or something?

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