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substitution in expression

asked 2013-05-17 23:57:53 -0500

updated 2013-05-18 03:23:18 -0500

Hi,

After reading Simplify shenanigans, I am very confused about the possibility of doing

sage: sin(x).subs(sin(x) == 3)
3

Which becomes even more funny with

sage: SR(2).subs(SR(2) == pi)
pi

I thought one may only set values to variable but no: I can set value to integers !

Is there a concrete example where this is useful ? Or this is just one more argument to avoid the use of the symbolic ring ?

Anyway the substitution is not at all consistent with the behavior of the symbolic ring. If such substitutions are allowed, then the expression SR(2) + SR(2) should not be simplified !

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Perhaps such a feature could be used for the [variation of constants](http://en.wikipedia.org/wiki/Variation_of_constants) method :p

tmonteil gravatar imagetmonteil ( 2013-05-19 00:52:42 -0500 )edit

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answered 2013-05-22 00:12:29 -0500

The substitute command is a generic interface to the rewrite engine in Pynac/GiNaC. You can replace an arbitrary subexpression, also containing wildcards (see the documentation of the match() method of symbolic expressions), with another expression.

Consider the case where a subexpression occurs in several places and you have a better formula or value for it derived from other sources. You can just replace the existing subexpression with the new formula using the substitute() function.

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Thanks for the precision of the implementation. And I see how it may be used.

vdelecroix gravatar imagevdelecroix ( 2013-05-22 02:37:47 -0500 )edit

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Asked: 2013-05-17 23:57:53 -0500

Seen: 1,246 times

Last updated: May 22 '13