# problem doing implicit plotting

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Chris on 11 Jul 2024 at 23:57
Commented: Chris on 12 Jul 2024 at 18:23
Hi, I am trying to make an implicit 3D plot in Matlab. A Matlab Help page says to use the following (as an example):
f = @(x,y,z) x.^2 + y.^2 - z.^2;
interval = [-5 5 -5 5 0 5];
fimplicit3(f,interval)
This works fine. But if I try to complicate things a bit by changing f to:
f = @(x,y,z) x.^2 + y.^2 - z.^2 + x.*y.
then I get the error message "Error: Invalid expression. Check for missing or extra characters".
Any idea what the problem is?
Thank you!

Torsten on 12 Jul 2024 at 14:41
Moved: Torsten on 12 Jul 2024 at 14:42
Remove the dot at the end of the expression x.*y.
Sam Chak on 12 Jul 2024 at 18:08
@Chris, you can see the effect of the dot in this example and comparison.
x = [1 2;
3 4];
% Case 1: element-wise product
y = x.^2
y = 2x2
1 4 9 16
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% Case 2a: 2x2 matrix multiplication
y = x^2
y = 2x2
7 10 15 22
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% Case 2b: 2x2 matrix multiplication
y = [x(1)*x(1) + x(2)*x(3), x(3)*x(1) + x(4)*x(3)
x(1)*x(2) + x(2)*x(4), x(3)*x(2) + x(4)*x(4)]
y = 2x2
7 10 15 22
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Chris on 12 Jul 2024 at 18:23
Ahhh, I see. Thank you!
C