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I have a little problem; I have defined the following Piecewise-function in the variables {x,L,w}:

f[x_, L_, w_] := Piecewise[{{x w (L - x), 0 <= x <= L}}];

Then I have defined the normalization of this function:

norm[L_, w_] := Integrate[f[x, L, w]^2, {x, 0, L},Assumptions -> (L > 0) \[And] (w > 0) \[And] (L >= x >= 0)];

(I have tried different forms of 'Assumptions' but the following problem persists). Then I defined the normalized function:

fnorm[x_, L_, w_] := f[x, L, w]/Sqrt[norm[L, w]];

When I tried to plot the previous normalized function, for some numerical values of variables {L,w}, many problems appears:

Plot[fnorm[x, 1, 1], {x, 0, 1}];
Integrate::ilim: Invalid integration variable or limit(s) in {0.0000204286,0,1}.
NIntegrate::itraw: Raw object 0.000020428571428571424` cannot be used as an iterator.
NIntegrate::itraw: Raw object 0.000020428571428571424` cannot be used as an iterator.
Integrate::ilim: Invalid integration variable or limit(s) in {0.0204286,0,1}.
NIntegrate::itraw: Raw object 0.02042859183673469` cannot be used as an iterator.
General::stop: Further output of NIntegrate::itraw will be suppressed during this calculation.
Integrate::ilim: Invalid integration variable or limit(s) in {0.0408368,0,1}.
General::stop: Further output of Integrate::ilim will be suppressed during this calculation.

Obviously the plot of the normalized function dosen't appeare. The plot,for a range of numerical value for {L,w}, of the normalization, appears:

Plot3D[norm[L, w], {L, 0.5, 1}, {w, 0.5, 1}]

The previos plot appears without problems.

Thanks for any tips and helps!

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  • $\begingroup$ Plot set x equal to a value, then calls fnorm[]. Try Evaluate[fnorm[x, 1, 1]]. — Somewhere on site this question is already answered in full. I’ll try to find it. $\endgroup$
    – Michael E2
    Nov 27, 2021 at 13:53
  • $\begingroup$ E.g. mathematica.stackexchange.com/a/255208/4999, mathematica.stackexchange.com/q/124706/4999, mathematica.stackexchange.com/q/46385/4999 $\endgroup$
    – Michael E2
    Nov 27, 2021 at 13:55
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    $\begingroup$ One could also use = instead of := in defining norm[], since the symbolic integral can be done: norm[L_, w_] = Integrate[..]. If you want to keep :=, then best practice would be to localize x: norm[L_, w_] := Module[{x}, Integrate[..]]. The use of SetDelayed (:=) means the integral is recalculated every time norm[] is called, which would make the code quite slow probably. $\endgroup$
    – Michael E2
    Nov 27, 2021 at 14:17

1 Answer 1

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Redefine your functions by using different argument names:

f[x_, L_, w_] := (Print["x=", x]; 
  Piecewise[{{x w (L - x), 0 <= x <= L}}]); 
norm[L_, w_] := 
 Integrate[f[x1, L, w]^2, {x1, 0, L}, 
  Assumptions -> (L > 0) \[And] (w > 0) \[And] (L >= x >= 0)];
fnorm[x2_, L_, w_] := f[x2, L, w]/Sqrt[norm[L, w]];

Now, if we run this:

Plot[fnorm[x3, 1, 1], {x3, 0, 1}]

You get the output:

enter image description here ....

You see that Integrate and Plot call their functions with symbolic arguments. This is done to eventually simplify the expressions. But in your case "f" delivers not a number as result if called with a symbolic argument. Therefore, ensure that "f" is only called on numeric arguments by:

Clear[f, norm, fnorm]
f[x_?NumericQ, L_, w_] := Piecewise[{{x w (L - x), 0 <= x <= L}}]; 
norm[L_, w_] := 
 Integrate[f[x1, L, w]^2, {x1, 0, L}, 
  Assumptions -> (L > 0) \[And] (w > 0) \[And] (L >= x >= 0)];
fnorm[x2_, L_, w_] := f[x2, L, w]/Sqrt[norm[L, w]];
Plot[fnorm[x3, 1, 1], {x3, 0, 1}]

enter image description here

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  • $\begingroup$ Thanks for the answer! It works perfectly now. $\endgroup$
    – Lorenzo Bagnasacco
    Nov 27, 2021 at 14:26

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