Can't get my Maclaurin Series function to use symsum properly?

Here is the code I'm working with currently:

function [ output_args ] = MaclaurinSeries( f,a,tol )
syms x k;
y = subs(f,a)
disp(f)
i=1
error = 1;
while error > tol
    f1 = symsum(diff(f,x,k) * (x^k)/(factorial(k)),k,0,i)
    error = y - subs(f1,a)
    q = subs(f1,a)
    i=i+1
end

Every time I run it, no matter how many times I tweak it, I manage to get f1 = 0 every time (which just isn't true). It calculates the derivative properly and everything, but I just don't know why it's returning zero. Any help?

Answer 1

[Walter Roberson]

symsum(diff(f,x,k) * (x^k)/(factorial(k)),k,0,i)

is processed as

temp = diff(f,x,k) * (x^k)/(factorial(k));
symsum(temp, k, 0, i)

That is, the expression is evaluated first (as is the case for all MATLAB function arguments) and the result of the expression has the symsum applied to it.

But diff(f, x, k) with symbolic k is 0 . I do not know why they made that choice.

With it coming out as 0, you are doing symsum(0, k, 0, i) which is going to be 0.

I would advise not using symsum for this. Instead, keep a running total, adding one more term at each point.

[Cole Butler]

I'm not sure what you mean by a running total

[Torsten]

You are not allowed to use MATLAB's "taylor" ?

Best wishes

Torsten.

[Walter Roberson]

f1 = sym(0);
while ...
  new_term = ...
  f1 = f1 + new_term;
  ...
end