# [ExI] knees and math: was RE: How Infinite Series Reveal the Unity of Mathematics

spike at rainier66.com spike at rainier66.com
Mon Jan 24 16:48:19 UTC 2022

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-----Original Message-----
From: spike at rainier66.com <spike at rainier66.com>
Sent: Monday, January 24, 2022 7:56 AM
To: 'ExI chat list' <extropy-chat at lists.extropy.org>
Cc: spike at rainier66.com
Subject: RE: [ExI] How Infinite Series Reveal the Unity of Mathematics

-----Original Message-----
From: extropy-chat <extropy-chat-bounces at lists.extropy.org> On Behalf Of BillK via extropy-chat
Subject: [ExI] How Infinite Series Reveal the Unity of Mathematics

Infinite sums are among the most underrated yet powerful concepts in mathematics, capable of linking concepts across math’s vast web.
By Steven StrogatzJanuary 24, 2022

<https://www.quantamagazine.org/how-infinite-series-reveal-the-unity-of-mathematics-20220124/>

Quote:
The most compelling reason for learning about infinite series (or so I tell my students) is that they’re stunning connectors. They reveal ties between different areas of mathematics, unexpected links between everything that came before. It’s only when you get to this part of calculus that the true structure of math — all of math — finally starts to emerge.
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Fascinating story!   Recommended!

BillK

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>...Great article BillK thanks!

>From BillK's Strogatz article:

>...which tells us a = 35 × 20 = 12 miles. Similar reasoning reveals that the legs shrink by a ratio of r= 15 each time the fly turns around. Von Neumann saw all of this instantly and, using the a/(1–r) formula above, he found the total distance traveled by the fly:

>...S = 121−15 = 1245 = 604 = 15 miles...

It is entirely plausible that Von Neumann really did use the infinite series approach to solve the fly problem.  Engineers will nearly always see the shortcut but mathematicians know the hell outta their infinite series formulas and tend to think in those terms.  My son and I had a similar experience with the fly problem: he solved in about a minute using the a/(1–r) formula.  I got it in about 10 seconds using engineering insight.

Speaking of which... a new puzzle (of sorts) occurred to me this morning as my teenage mathematician bounded down the stairs baDOOMP ba DOOMP baDOOMP and the thought came to mind: I don't bound down the stairs like that.  So... why don't I bound down the stairs like that?  Because it hurts my knees to bound down like that.  So... why does it hurt my knees but not his, and why didn't it hurt mine thirty years ago?  If I damaged my knees with running, well... that was over with thirty years ago.  I stopped running daily about in my early 30s, but now I am paying the price for that youthful indiscretion.  But thirty years ago, after I stopped running, I would still bound down the stairs without pain.

For the last 30 years, I have taken good care of my knees, but still they remind me early and often of the decades of youthful indiscretion, of the common paradigm of the day: no pain no gain.  OK, we can arrange pain.  In retrospect it should have been no brain no gain, but we didn't know.  We thought it had to hurt to do any good.  So... we made it hurt.  Now we don't want it to hurt anymore, and even if it does, it appears to be a no gain anyway.

So... why do old knees hurt but young knees do not?  What physiological change took place?

spike

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