how to make this summation

[John D'Errico]

With a loop? Note that variables in MATLAB cannot be indexed with negative numbers.

[mohamed enan]

with a loop or symsum anything that makes this work

[John BG]

Mohamed are you calculating DTFT coefficients?

the[-3:1:3] window is shifting isn't it?

can you supply the whole chain of ak coefficients?

[John BG]

Dear Mr D'Errico

YES

in MATLAB the index to matrices cannot be 0 or negative, .

BUT .

NO,

you can build an index vector to make the k shift along any desired values, so ONE can use negative indices, not directly, but just check what the Image Analyst wrote below.

Answer 1

[Image Analyst]

Perhaps this:

j = sqrt(-1)
a = rand(1,7); % Whatever...
omega = 2 % Whatever....
t = 42; % Whatever t value you want to evaluate x at.
k = -3 : 3;
x = sum(a .* exp(j .* k .* omega * t))

[Jan]

I think 1j is nicer than redefining j=sqrt(-1).

[Image Analyst]

You're right. That's a nice shortcut (if one remembers it).