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arithmetic with matrices of formal functions

asked 2013-08-19 08:31:38 -0500

Mark gravatar image

If I have two plain formal functions, like

sage: var('q k')
sage: function('u')
sage: function('v')

it seems I can always get more 'involved' formal functions, like the following h(k,q), by symbolic operations

sage: h(k,q)=u(k)*v(q)
sage: h(17,sin(k))
u(17)*v(sin(k))       # OK.

My naive hope was, that this would straightforwardly apply also to matrix construction from formal functions, like so

sage: B(k,q) = matrix([[u(k),v(q)],[v(q),u(k)]])
# but
sage: B(17,sin(q))
[u(k) v(q)]
[v(q) u(k)]       # :(

So, B(k,q) does not substitute the variables. After some attempts, it seems that this is a valid way to construct a callable symbolic matrix from formal functions

sage: B=matrix(CallableSymbolicExpressionRing((k,q)),[[u(k),v(q)],[v(q),u(k)]])
# at least
sage: B(x^2,cos(q))
[   u(x^2) v(cos(q))]
[v(cos(q))    u(x^2)]        # OK.

My 1st question is, if there is not a 'less cryptic' way to construct B from u and v?

My 2nd question concerns doing matrix arithmetic with B. If I try to do something similar to that for the plain formal function arithmetic for h(k,q) with my formal matrix function B it 'does not work'. Eg.:

sage: H(k,q)=B(k,q)*B(q,q)
sage: H(sin(17),pi)
[   u(k)*u(q) + v(q)^2 u(k)*v(q) + u(q)*v(q)]
[u(k)*v(q) + u(q)*v(q)    u(k)*u(q) + v(q)^2]  # :(

I.e., again, upon matrix multiplication, the object B(k,q)*B(q,q) seems to not substitute the arguments into the formal functions anymore. However, if I do

sage: H=matrix(CallableSymbolicExpressionRing((k,q)),B(k,q)*B(q,q))


sage: H(sin(17),pi)
[    u(pi)*u(sin(17)) + v(pi)^2 u(pi)*v(pi) + u(sin(17))*v(pi)]
[u(pi)*v(pi) + u(sin(17))*v(pi)     u(pi)*u(sin(17)) + v(pi)^2]    # OK.

But it seems awkward if one would have to do matrix arithmetic like that. So, how does one do proper matrix operations with matrices made from formal functions?

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answered 2013-08-23 20:58:41 -0500

nbruin gravatar image

The problem arises from the fact that matrices with symbolic expressions are not "symbolic expressions" themselves:

 sage: B = matrix([[u(k),v(q)],[v(q),u(k)]])
 sage: type(B)
 sage: type(u)
 sage: type(u(q))

and hence, trying to turn it into a CallableSymbolicExpression by itself fails. Possibly the easiest thing is to just live with B. You can still substitute values, as long as you're explicit about the substitutions:

sage: B(k=17,q=sin(q))
[    u(17) v(sin(q))]
[v(sin(q))     u(17)]

The nice thing is that you can do matrix algebra with these:

sage: H=B(k=k,q=q)*B(k=q,q=q)
sage: H
[   u(k)*u(q) + v(q)^2 u(k)*v(q) + u(q)*v(q)]
[u(k)*v(q) + u(q)*v(q)    u(k)*u(q) + v(q)^2]
sage: H(k=sin(17),q=pi)
[    u(pi)*u(sin(17)) + v(pi)^2 u(pi)*v(pi) + u(sin(17))*v(pi)]
[u(pi)*v(pi) + u(sin(17))*v(pi)     u(pi)*u(sin(17)) + v(pi)^2]

Presently, "callable" symbolic expressions and "symbolic" matrices are two extensions built on top of symbolic expressions and they don't mix very well.

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Asked: 2013-08-19 08:31:38 -0500

Seen: 102 times

Last updated: Aug 23 '13