I use quaternions and Clifford algebras frequently for solving PDE boundary value problems as well as things like reflections, rotations etc. The underlying field for my quaternions is almost always either the complex numbers or the symbolic ring.
I am making an attempt to use them in Sage but I've run into a couple of obstacles that have given me pause. The absolute first things I looked for were,
1) scalar part or real part of the quaternion. Where is this function? I stumbled upon turning it into a list or vector but I presume that just taking the scalar part would be more efficient than dumping all the coefficients. For matrix representations it's often just the trace of the matrix. Correspondingly obtaining the vector part should be a standard function as well.
2) Quaternion automorphisms and anti-automorphisms? where are they? Yes we have conjugate which is both a method and an external function but what about the reversion and involution and the many other automorphisms? I don't expect to have to multiply by a bunch of unit vectors to get them for efficiency reasons. Also how do I distinguish between the quaternion conjugate and the complex conjugate of each of it's elements? This is a very important distinction that I would not know how to do without stripping out it's components and then remapping it.
3) Constructing a quaternion from coefficients. The only examples I've seen require explicit multiplications like q = q0 + q1 * i + q2 * j + q3 * k . That doesn't seem efficient.
4) quaternions as elements of enclosing matrices and vectors. This would be very helpful since you could generate any Clifford algebra with this and it's often easier to analyze the components of said algebra that are isomorphic to Quaternions or Biquaternions in the complex case. Moreover I need to be able to do this for quaternions over the symbolic ring. It fails for me as this example from sage 7.3 shows:
Q.<e1,e2,e3> = QuaternionAlgebra(SR, -1,-1)
var('x y z', domain='real')
q1 = x + x * I * e1
q2 = x - x * I * e1
v = vector([q1,q2])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "_sage_input_36.py", line 10, in <module>
exec compile(u'open("___code___.py","w").write("# -*- coding: utf-8 -*-\\n" + _support_.preparse_worksheet_cell(base64.b64decode("US48ZTEsZTIsZTM+ID0gUXVhdGVybmlvbkFsZ2VicmEoU1IsIC0xLC0xKQp2YXIoJ3ggeSB6JywgZG9tYWluPSdyZWFsJykKcTEgPSB4ICsgeCAqIEkgKiBlMQpxMiA9IHggLSB4ICogSSAqIGUxCnYgPSB2ZWN0b3IoW3ExLHEyXSk="),globals())+"\\n"); execfile(os.path.abspath("___code___.py"))
File "", line 1, in <module>
File "/tmp/tmpQ0ibdW/___code___.py", line 7, in <module>
exec compile(u'v = vector([q1,q2]) File "", line 1, in <module>
File "sage/modules/free_module_element.pyx", line 510, in sage.modules.free_module_element.vector (/data/AppData/SageMath/src/build/cythonized/sage/modules/free_module_element.c:5811)
TypeError: unsupported operand type(s) for ** or pow(): 'QuaternionAlgebra_ab' and 'int'
5) quaternion rotations. Most libraries will generate the unit quaternion that rotates in 3D space and/or have a function that applies it efficiently.
There are probably a few other things like generating a canonical matrix representation for a full quaternion and so forth. I actually wonder if the default quaternion package is the right tool for a physicist or an engineer or someone who wants to play around with the so called geometric algebra of Hestenes. It seems the perogatives of the scientist vs the algebraist are very very different. How efficient is this library? What if I had to multiply millions of quaternions? I'm guessing I'd be better off mapping it to complex matrices and invoking blas.
While most of the functionality can be added on via a little python programming, I am concerned about efficiency and consistency. Moreover item 4 is a bit of a quandary since it involves overloading a bunch of arithmetic operators and some new python classes and so forth.
I'm hoping someone will tell me that it's all there, I just wasn't able to find it in the documentation or so forth. Thanks for any help in advance.
