Derivative of a function in a particular point

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Hi
I have an external function y= function fa(x)
y=sin(x./pi)
end
i want in the main program to find the derivative of fa in numerical point
thank you
George

Accepted Answer

george veropoulos
george veropoulos on 16 Jul 2021
format long
x = 2 ;
h = sqrt(eps) ;
f1=(sin(x + h)./(x+h) - sin(x)./x) / h % -0.400000
% -0.435397773981094
f2=cos(x)./x-sin(x)./x^2
% -0.435397774979992
  2 Comments
Walter Roberson
Walter Roberson on 16 Jul 2021
format long
x = 2 ;
h = sqrt(eps(x)) ;
f1=(sin(x + h)./(x+h) - sin(x)./x) / h
f1 =
-0.435397778831590
f2=cos(x)./x-sin(x)./x^2
f2 =
-0.435397774979992

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More Answers (1)

Jan
Jan on 16 Jul 2021
Use one of the quotients of differences to get a numerical approximation of the derivative:
x = 1.2345;
h = sqrt(eps);
dy_right = (fa(x + h) - fa(x)) / (h)
dy_left = (fa(x) - fa(x - h)) / (h)
dy_both = (fa(x + h) - fa(x - h)) / (2 * h)
function y = fa(x)
y = sin(x./pi)
end
  9 Comments
Jan
Jan on 18 Jul 2021
@george veropoulos: The numerical analysis for the optimal choice of h is still worth to write a PhD thesis. As said before, the 2nd derivative of the function matters. To estimate this, you need a further small variation. Because this is expensive, if the function to be evaluated is huge, some heuristics are useful. This can be an important part of the processing time if you optimize an expensive function. A related field is the optimal choice of temproal and spatial steps sizes in finite element problems, e.g. the simulation of the earth clima.
The rule of thumb: If the function and the argument are about 1 and do not explode nearby, sqrt(eps) is a fair choice between the cancellation and discretization error. The "fairness" can be checkd by testing 10*sqrt(eps) and 0.1*sqrt(eps): If they reply the same derivative, you can assume to be on the right side. For a professional simulation, the explanation must sound more seriously. ;-)
george veropoulos
george veropoulos on 19 Jul 2021
thank you all! very helpfull dicussion
George

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