Re: Newbie Limit problem

• To: mathgroup at smc.vnet.net
• Subject: [mg53479] Re: Newbie Limit problem
• From: Paul Abbott <paul at physics.uwa.edu.au>
• Date: Thu, 13 Jan 2005 03:59:53 -0500 (EST)
• Organization: The University of Western Australia
• Sender: owner-wri-mathgroup at wolfram.com

```In article <cs5bce\$3tk\$1 at smc.vnet.net>,
Ken Tozier <kentozier at comcast.net> wrote:

> I'm self taught so I wasn't aware of the official name of the class of
> curves I'm looking at, but have since found that they are called
> "trochoids" and they have a whole section devoted to them here:
> http://mathworld.wolfram.com/Trochoid.html. Integrals might as well be
> written in Martian for all the meaning I get out of them. I find them
> impenetrable, so I don't know if the trochoids  arc length formula
> here: http://mathworld.wolfram.com/CurtateCycloid.html is considered
> "closed form" or not.

Equation (3) at this page is a closed form expression for the arc length
and Mathematica can compute the complete elliptic integral of the second
kind and Jacobi elliptic functions to arbitrary precision. However, it
is not obvious to me how the sum you wrote down, essentially

Sum[Sqrt[ (d^2 + 2 s^2 -  2 s (s Cos[(2 Pi)/s] +
d (Cos[(2 k Pi)/s] - Cos[(2 (1 + k) Pi)/s])))]/s,
{k, 0, Infinity}]

relates to the arc length. I assume this is some sort of Riemann sum
approximation to the integral? Note that there is a Mathematica Notebook
at http://mathworld.wolfram.com/CurtateCycloid.html and in that Notebook the
arc length is computed in closed form using Mathematica as

2 (a - b) EllipticE[t/2, - 4 a b/(a - b)^2]

for a > b.

Cheers,
Paul

--
Paul Abbott                                   Phone: +61 8 6488 2734
School of Physics, M013                         Fax: +61 8 6488 1014
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Crawley WA 6009                      mailto:paul at physics.uwa.edu.au
AUSTRALIA                            http://physics.uwa.edu.au/~paul

```

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