DISREGARD: math and the mother church

[Nick Thompson]

Russell,

Remember, mine was a book for English Majors,  Berlinski's Tour of the
Calculus.  

But thou quibblest! Dothn't thou? Why is the algebra necessary at all.
Doesnt the mean value theorem fall out of the definition of a mean and the
definition of continuity?  Full stop.  Granting only that the mean falls
between (or is one of) the extremes?  

Nick

PS.  I apologize for my message garblement.  In fact I had NOT sent an
incomplete message.  So the message saying "disregard the message" was the
only message.   "This is not a pipe."

Nick




> [Original Message]
> From: Russell Standish <r.standish at unsw.edu.au>
> To: <nickthompson at earthlink.net>; The Friday Morning Applied Complexity
Coffee Group <friam at redfish.com>
> Date: 7/22/2007 7:04:29 PM
> Subject: Re: [FRIAM] DISREGARD: math and the mother church
>
> On Sun, Jul 22, 2007 at 06:12:58PM -0600, Nicholas Thompson wrote:
> >
> > So, here is my present understanding of the mathematician's argument
for the mean value theorem.  What I dont understand is why it takes three
pages of algebra to get there!
>
> I don't know where you get the 3 pages from. My analysis book does it
> 2 paragraphs of algebra, half a page at most. (That's including all
> the necessary lemmas and definitions).
>
> >
> > Let us amagine that ab is a bit of a line.  It could be straight, and
the argument would still hold, but let us imagine that it is curved....
curved up, curved down, it does not matter.  Let's imagine that is an
inverted U, except that it doesnt have to be a straight up and down
inverted U.  In fact, it can be sitting so that somebody wobbled it so that
it is, at the instant of being photographed, standing on one leg, about 30
degrees from the verticle.  .  
> >
> > What does matter is that the line be continuous ALL THE WAY FROM a to
B.  No gaps,  not steps.  Imagine that no matter how small the steps you
are taking, you can walk along the points of the line from a to b and not
get your feet wet, NOT AT ALL -- if of course your shoe size is small
enough.  
> >
> > Now draw a line that connects the bottom of the two legs of the
inverted U.  As we just said, that line will move off to the right, from a
through b and beyond,  at about a thirty degree angle from the horizontal.
Thus the mean slope of the tilted inverted U is 30 degrees, right?
> >
> > Here is what that means, as I understand it. Every point on the tilted
inverted U has a "slope", the slope of the line that is just tangent to the
U at that point.  Near point "a" that slope is VERY positive;  near point
"b", that slope is very negative.  Now, imagine  you set out to walk along
the curve from "a" to "b".  If you take tiny enough steps, you MUST step on
the point where the slope is the same as the mean slope.   That is what the
mean value theorem says.  
> >
> > But I just got there without any of the algebra usually devoted to that
proof.  So the question is,  what is the VALUE of the algebra.  If one can
estab lish the truth of such an important MATHEMATICAL theorem in other
than mathematical means, what is the value of the maths?  

> >
>
> What you have given is the "handwaving" version of the proof. The
> trouble is that human imagination can easily get us into trouble when
> dealing with infinities, which is necessarily involved in dealing with
> the concept of continuity. In the above example, you mention that
> continuity is important, but say nothing about differentiability. Are
> you aware that continuous curves that are nowhere differentiable
> exist? I fact most continuous curves are not differentiable. By most,
> I mean infinitely more continuous curves are not differentiable than
> those that are, a concept handled by "sets of measure zero".
>
> To give an example, consider your interval joined by two line segments
> so as to form a single kink in the middle at point c:
>
> At all points on the interior, except for the c, the slope is either
> s1 = (f(c)-f(a))/(c-a) or s2=(f(b)-f(c))/(b-c). At c, the slope is
> undefined. But neither s1 nor s2 = (f(b)-f(a))/(b-1), so the mean
> value theorem fails.
>
> > I promise I am not MERELY trying to be a horses ass, here.  
> >
> > Nick
> >
> >
>
> Handwaving arguments are good for developing intuition. Great for
> teaching during a lecture, and get the students to study the rigorous
> proof later. Similarly, they're good for scientific seminars, but not
> scientific papers.
>
> Cheers
>
> >
> >
> >
> >
> >
> >  
> >
> >  
> >
> >  
> >
> > .  
> >
> >
> >
> >
> > Nicholas S. Thompson
> > Research Associate, Redfish Group, Santa Fe, NM (nick at redfish.com)
> > Professor of Psychology and Ethology, Clark University

(nthompson at clarku.edu)
> > ============================================================
> > FRIAM Applied Complexity Group listserv
> > Meets Fridays 9a-11:30 at cafe at St. John's College
> > lectures, archives, unsubscribe, maps at http://www.friam.org
>
> --
>
>
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