How can we compute the derivative of polynomials?

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Omar B. 2020-10-1

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How can we compute the derivative of the (m-1) polynomials at a point?

I got the following error:

The following error occurred converting from sym to double:

Unable to convert expression into double array.

Error in untitled (line 42)

p(1)=((x-alpha(1))*p0)/beta(1);

syms x 
 p0=1;
x0=2;
p(1)=((x-.2)*p0)/.3;
p(2)=((x-.5)*p(1)-.3*p0)/1.3;
dp(1)=eval((subs(diff(p(1),x,1),x,x0)) )
dp(2)=eval((subs(diff(p(2),x,1),x,x0)) )
for j=3:m-1
    p(j)=((x-.2)*p(j-1) - .3*p(j-2))/1.3;
    dp(j)=eval((subs(diff(p(j),x,1),x,x0)));
end

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James Tursa 2020-10-2

Both eval( ) and double( ) work for me with your example code. What is your actual code?

Omar B. 2020-10-2

yes, I got the same results. Thank you so much

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回答（1 个）

David Hill 2020-10-1

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Why not just use polynomial representation

p = [3 0 -2 0 1 5];
pd=polyder(p);
x=1:10;
val=polyval(pd,x);

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Omar B. 2020-10-1

I just here wrote an example , but I need to refer the coefficients to another code. Is there another way?

Walter Roberson 2020-10-2

The coedfficients would be in pd

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