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MathGroup Archive 2006

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Re: Errors at discontinuities

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65151] Re: [mg65110] Errors at discontinuities
  • From: "David Park" <djmp at earthlink.net>
  • Date: Wed, 15 Mar 2006 23:59:25 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

I think it is usually better practice to disentangle plotting from
derivation. Prepare the function you are going to plot outside of the
plotting statement. Then you can examine it, see if it is the form you want,
do test evaluations and more easily correct things if necessary.

a= 4 ; b = 3 ; tmax=10;
fn[u_,lam_,A_]=A*ArcSin[Sin[ 2 Pi u/lam]]*2/Pi ;

df[t_, a_, b_, c_, d_] = D[fn[t, a, b], t]/D[fn[t, c, d], t]

Plot[df[t, 3, 4, 5, 5.65], {t, 0, tmax}];

Also, in your plots c and d are never defined. I'm not certain if that
completely answers your question, but at least we obtain a plot.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/



From: Narasimham [mailto:mathma18 at hotmail.com]
To: mathgroup at smc.vnet.net


Errors appear at start / period cross-over points of  a triangular wave
on t-axis  with  period a, amplitude b. Why does not
Plot[  D[fn[t,3,4],t]  / D[fn[t,5,5.65],t] , {t, 0,tmax}] ; Or
Plot[  D[fn[t,3,4],t]  , {t, 0,tmax}] ;
work at beginning when the same works erroneously later on in sl1 and
gets self-corrected  at sl2? And how to remedy the situation?  TIA

a= 4 ; b = 3 ; tmax=10;
fn[u_,lam_,A_]=A*ArcSin[Sin[ 2 Pi u/lam]]*2/Pi ;
Plot[ fn[t,a,b], {t, 0,tmax}] ;
Plot[  D[fn[t,a,b],t]  / D[fn[t,c,d],t] , {t, 0,tmax}] ;
sl1=D[fn[t,a,b],t]/D[fn[t,c,d] ] ; Plot[ sl1 ,  {t, 0, tmax}] ;
sl2= D[fn[t,a,b],t]/D[fn[t,c,d],t] ; Plot [sl2, {t, 0, tmax}] ;



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