How to check if a symbolic function is positive or negative?
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Andrea Strappato
on 29 Sep 2021
Commented: Andrea Strappato
on 29 Sep 2021
Hi everyone,
I've defined a square function this way:
sym x
y = x^2
Running the command ' isAlways(y>=0) ' I get:
Warning: Unable to prove '0 <= x^2'.
> In symengine
In sym/isAlways (line 42)
Why Matlab is displaying the warning?
Thanks in advance for who will help me!
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Accepted Answer
Steven Lord
on 29 Sep 2021
MATLAB is unable to prove that y is always greater than or equal to 0 because y is not always greater than or equal to 0. Here is a case where it is:
x = 1;
y = x^2
y >= 0 % true
Here is a case where it is not.
x = 1i;
y = x^2
y >= 0 % false
If you told MATLAB that x was real, it would help.
syms x real
y = x^2
isAlways(y >= 0)
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