Plotting a function f(x) in Igor with known parameter values

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Tue, 07/10/2012 - 11:57 pm

Dear All

Suppose I have a function f(x), f(x)= A*exp[-(B^2*x^2)/3].

And I know the values of the parameters A, B.

How do I, in Igor, plot this function f(x) from X=0 to x=infinity in order to determine the value of f(0)?
Hi,

you can have a try with the Function Grapher in Analysis->Packages->Function Grapher.

bye, thomas

PS: Isn't it just f(x=0) = A*exp(0) = A?

July 11, 2012 at 01:51 am - Permalink

Igor_user wrote:
...How do I, in Igor, plot this function f(x) from X=0 to x=infinity in order to determine the value of f(0)?


variable A=1, B=2
make/N=100 fx
setscale/I x 0, 10^6, "", fx
fx = A*exp(-(B*x)^2/3)
display fx


FWIW, setting the end point of the scale range on a data wave to infinity is rather problematic if not impossible to handle.

--
J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAHuntsville

July 11, 2012 at 05:11 am - Permalink

thomas_braun wrote:
Hi,

you can have a try with the Function Grapher in Analysis->Packages->Function Grapher.

bye, thomas

PS: Isn't it just f(x=0) = A*exp(0) = A?


Thanks!

I tried to make this expression as simple as possible just for the readers.

The actual equation involves some unknown constants before A and that have to be determined through an extrapolation of this function to f(0).

July 11, 2012 at 07:55 pm - Permalink

Igor_user wrote:
I tried to make this expression as simple as possible just for the readers.

Then I would say you cut out the interesting part :)
Quote:
The actual equation involves some unknown constants before A and that have to be determined through an extrapolation of this function to f(0).

You still haven't really given us sufficient detail. It sounds like it might be a job for curve fitting, which can find optimum values of unknown parameters if you have a data set that is modeled by the function.

Maybe you should provide some details about exactly what you're trying to do.

John Weeks
WaveMetrics, Inc.
support@wavemetrics.com

July 12, 2012 at 09:10 am - Permalink

johnweeks wrote:
Igor_user wrote:
I tried to make this expression as simple as possible just for the readers.

Then I would say you cut out the interesting part :)
Quote:
The actual equation involves some unknown constants before A and that have to be determined through an extrapolation of this function to f(0).

You still haven't really given us sufficient detail. It sounds like it might be a job for curve fitting, which can find optimum values of unknown parameters if you have a data set that is modeled by the function.

Maybe you should provide some details about exactly what you're trying to do.

John Weeks
WaveMetrics, Inc.
support@wavemetrics.com


Ok,

The full function is expressed by this equation.

I(Q) = (N_p)*(V_p)^2*(D_sig)^2*G*exp[-(R_g)^2*(Q^2)/3]

Now, I have a data which is fitted by this function,

G*exp[-(R_g)^2*(Q^2)/3]

and I known G, (R_g), (D_sig) and I can work out (V_p) using (R_g).

But I have to determine (N_p) which requires the value of I(Q) at Q=0.

The problem is that my data doesn't cover Q=0. So, it requires some linear extrapolation at small Q to get the I(0) value.

How should I do that in Igor?

July 17, 2012 at 02:41 am - Permalink

The attached experiment might help. The wave IQ has simulated data starting at Q=1. The fit is made to the function N_p*exp(-(R_g*Q)^2/3), with the fit range extending to the full range of the graph (0 to 10). The returned coefficients give N_p and R_g.

--
J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAHuntsville
ExampleFit.pxp

July 17, 2012 at 05:36 am - Permalink