# Truncation error plot with weird issue

I have a function f(x) = sin(x)/x^3 whose first derivative I am trying to estimate using 1st, 2nd and 4th order Finite Difference schemes. I tried to plot the truncation error in MATLAB and I found myself with increasing error with smaller step size. I am not sure where I am wrong. Attached is my code snippet.

syms y
dx = linspace(10^(-5), 10^(-0));
f = @(x) sin(x)/x^3;
x = 4;

df1 = (f(x+1)-f(x))./dx;
df2 = (f(x+1)-f(x-1))./(2.*dx);
df4 = (f(x-2)-8*f(x-1)+8*f(x+1)-f(x+2))./(12.*dx);

exact = cos(x)/x^3 - (3*sin(x))/x^4;

for i=1:length(dx)
error1(i) = abs(df1(i)-exact)
error2(i) = abs(df2(i)-exact)
error4(i) = abs(df4(i)-exact)
end

figure
loglog(dx,error1,'r','linewidth',2)
hold on
loglog(dx,error2,'b','linewidth',2)
loglog(dx,error4,'k','linewidth',2)

• Found the problem. I forgot to multiply by dx inside the difference. – Yukti Kathuria Feb 17 '20 at 23:13
• Nice work! Probably worth posting the correct code and accepting your own answer. Otherwise the site will just continue to auto-bump your question, in an effort to attract answers. – rchilton1980 Feb 18 '20 at 3:04