[Maxima-bugs] [ maxima-Bugs-2014941 ] compositions of 'at'

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Bugs item #2014941, was opened at 2008-07-10 13:26
Message generated for change (Comment added) made by crategus
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Category: Lisp Core
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Private: No
Submitted By: Barton Willis (willisbl)
Assigned to: Nobody/Anonymous (nobody)
Summary: compositions of 'at'

Initial Comment:
(%i26) at(diff(f(x),x),[x=b]);
(%o26) at('diff(f(x),x,1),[x=b])

(%i27) at(%,[b=a]);
(%o27) at('diff(f(x),x,1),[x=b])

(%o27) should either be at('diff(f(x),x,1),[x=a]) or  at(at('diff(f(x),x,1),[x=b]),[b=a])

Also, I think 'at' should be a simplifying function.




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>Comment By: Dieter Kaiser (crategus)
Date: 2010-06-19 21:13

Message:
Fixed in comm2.lisp revision 1.38. The composition of AT is more correct
and the list of equations is sorted.

There is one open problem: AT doesn't check for the "freeof" case. This
topic needs more discussions and perhaps a new bug report.

Closing this bug report as fixed.
Dieter Kaiser

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Comment By: Barton Willis (willisbl)
Date: 2008-07-10 14:12

Message:
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And yet another thing: Maxima should sort the second argument to 'at.'
This
would allow more expressions to simplify to zero; example:

(%i29) diff(f(x,y),x,1,y,1);
(%o29) 'diff(f(x,y),x,1,y,1)

(%i30) at(%,[x=a,y=b]) - at(%,[y=b,x=a]);
(%o30)
at('diff(f(x,y),x,1,y,1),[x=a,y=b])-at('diff(f(x,y),x,1,y,1),[y=b,x=a])

Since 'at' does multiple substitutions in series, not parallel, the logic
could be 
complicated. Should the subs be done in parallel?

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Comment By: Barton Willis (willisbl)
Date: 2008-07-10 14:04

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Another thing: 'at' doesn't check for the "freeof" case; for example:

(%i24) diff(f(z),z);
(%o24) 'diff(f(z),z,1)
(%i25) at(%,[x=1]);
(%o25) at('diff(f(z),z,1),[x=1])



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