Talk:Change of variables

Error

There is an error at the "Simple example" section. It is written that x+y=55 xy=16 don't give a solution while they actually do. And the solution is (x;y)=(0.29246;54,70754). I've written it with some degree of accuracy, but I guess it is not OK to be written this way, but I think it's better than the statement that these equations have no solution at all! — Preceding unsigned comment added by EMoMaD (talk • contribs) 22:18, 16 September 2013 (UTC)

Request for Clarification

Can someone clarify the steps in the example in the Differentiation section? Cause I think I'm missing something during the breakdown of d/dx, specifically what happened to d/dx of x^2 and how that transformed into 2x. -- (Mrja84 (talk) 18:52, 2 August 2009 (UTC))

Intro

Why would someone make such a strange formula as an intro? —Preceding unsigned comment added by 128.250.5.246 (talk) 08:45, 9 March 2010 (UTC)

8th order polynomial?

In what sense is that an 8th order polynomial? The highest degree of any term is 6!

--watson (talk) 22:15, 27 March 2010 (UTC)

Merge proposal

It seems to me that the topic of the article Substitution of variables (a term I don't remember encountering before) is the same as the topic treated here.  --Lambiam 19:11, 14 September 2011 (UTC)

Yes, the terms "substitution of variables" and "change of variables" are used interchangably. The word substitution might be more formal. However, the articles presently are focused on applications in physics and algebra, respectively, and are categorized accordingly. Nevertheless, substitution of variables and change of variables should lead to the same article. Isheden (talk) 12:45, 18 October 2011 (UTC)

Agree that these terms are exact synonyms and have performed the merge. I did not merge the PDE version as that, like Integration by substitution, is a more specific topic. Since both pages were basically a list of examples, I just dragged them all over with minimal copyediting so we now have a longer list of examples. Kilopi (talk) 06:09, 23 January 2013 (UTC)

Complete roots

A 6th order polynomial has six roots. How can we find the complete roots by the proposed substitution? Only the roots x=1 and x=2 are calculated.--Kaktus Kid (talk) 20:24, 23 May 2012 (UTC)

Other solutions are complex and are equal to 3rd complex roots of 1 and 2. — Preceding unsigned comment added by 62.245.100.121 (talk) 14:40, 11 June 2012 (UTC)

Merge from substitution (algebra)

The article substitution (algebra) has been merged incorrectly to this one: The term "substitution" was not defined after the merge, and the tag "merge proposal" has not been removed. I have not reverted the merge, but I have included in the lead a definition of "substitution" and clarified its relation with change of variable. I have not a clear opinion on the best solution (one or two articles). D.Lazard (talk) 16:39, 7 January 2014 (UTC)

Introduction

I also think the intro needs a lot of work and clarification. It is really hard to get anything out of it. In particular, the first paragraph is hard to understand. Compare with Alpha:

In other words, an expression involving free variables may be considered as defining a function, and substituting values to the variables in the expression is equivalent to applying the function defined by the expression to these values.

v

A theorem which effectively describes how lengths, areas, volumes, and generalized n-dimensional volumes (contents) are distorted by differentiable functions.

The second is succinct, and gives an impression of how it is used, and doesn't take 10 minutes to parse. That being said, I am not an expert on calculus, so I need to appeal to others for clarification. — Preceding unsigned comment added by OceanEngineerRI (talk • contribs) 19:56, 8 July 2016 (UTC)

I agree that the article is a mess. This partly results from a three years old merge of Substitution (algebra), done by Michael Hardy. IMO, this merge was erroneous. It appears from the merge discussions that the merge proposal was to merge here substitution of variables. However, this was a redirect and the error was to merge the target Substitution (algebra) of substitution of variables, instead of simply changing the target of the redirect.

In fact, change of variables is an operation that is different of substitution, even if the operations are often equivalent. A substitution may be done only on free variables, as said in the lead, while changes of variable may occur on non free variables, for example in integrals. Thus a change of variables is not necessarily a particular type of substitution, contrarily of the first assertion of the second paragraph of the lead. On the other hand, substituting a number for a variable is a substitution that is not a change of variable. Moreover, substitution in expressions is a fundamental operation in most computer algebra systems.

Therefore, two separate article are needed. D.Lazard (talk) 09:29, 9 July 2016 (UTC)

Plug and chug is NOT a change of variables!!

The article plug and chug redirects here, but that is clearly wrong -- plug and chug is about the algorithmic solution to a problem, by carrying out proof steps -- its low-brow version of what would be called proof theory in high-brow terms. In low-brow terms, its what engineering students do, when they find some equation in a textbook, and then plug in numbers, pull out their (ahem) modern-day slide-rule and see where they end up. This clearly is not a change of variables! 67.198.37.16 (talk) 18:36, 26 September 2016 (UTC)

You are probably right. This is not a problem of this article, but a problem of plug and chug. As apparently you have some expertise on this subject, the best is that either you change the redirect to a better target, or you transform the redirect into an article or, at least, a stub (your post is a possible starting point for such a stub). D.Lazard (talk) 21:08, 26 September 2016 (UTC)