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Topic: What's wrong with cumtrapz?
Replies: 2   Last Post: May 4, 2001 10:09 AM

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Timothy E. Vaughan

Posts: 39
Registered: 12/7/04
Re: What's wrong with cumtrapz?
Posted: May 4, 2001 10:09 AM
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""Leung, Randolph [COPE/HKG]"" <> wrote in message
> I have some experimental time traces to integrate. CUMTRAPZ seems
> to be a simple and easy option for me. To test it, I tried to integrate a
> simple SIN(X) time trace as follows,
> x = 0:pi/100:4*pi];
> y = sin( 2*pi*x).
> inty = cumtrapz( x', y' );
> I would expect a COS(X) time trace after CUMTRAPZ. The integrated
> time trace gave a cos pattern of variation, correct amplitude BUT was
> wrong in phase and shifted upwards, i.e. it is greater than zero for all
> x and gives 0, rather than 1, at x = 0. I am very confused with the
> results.

It seems you may be forgetting a bit of your calculus. You are numerically
taking the INDEFINITE integral of your function. In that case, you must be
prepared to add a constant to your solution. [I think it is really only
shifted "upward", and not really wrong in phase.] You need to determine the
appropriate constant from other conditions of your problem.


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