Hi everyone, can someone help me with this double sum ?

Hi everyone, I am struggling on how to write this double sum in matlab, hoping someone can help me:

thanks in advance !!

[Torsten]

You cannot refer to a summation index j in the outer sum which is not yet defined.

So your double sum as written makes no sense.

Maybe you mean to flip the summation signs.

Answer 1

[John D'Errico]

Surely you can use a loop? Actually just nested loops? Perhaps your problem is MATLAB does not allow a zero index, yet you have what would seem to be a zero index.

Next, are those powers of S and T?

Anyway, all you need to do is

ThisSum = 0;
for j = 0:4
    for i = 0:4-j
        ThisSum = ThisSum + a(i+1,j+1)*T^i*S^j;
    end
end

I assume that a is an array, since you have what apear to be indices. And a would need to be a 5x5 array.

Are there other ways to solve this? Well, yes. Do you need them? NO.

[marco bosisio]

thank you very much for your quick answer,

the problem is that "a" is indeed an array but it is a 1x15 array, I am not sure on how to index it.

[John D'Errico]

How would I know either? Your formula refers to an array that appears to have TWO indices, i and j. But you tell us that a is just a 1x15 vector. Without knowing what the elements of a are, or what they mean, they are just a list of numbers. Perhaps it really was an array, in some unrolled form. But without knowing exactly what a contains, or why it is that shape, we are left with no answer. A list of numbers contains no meaning associated with it. The meaning can only be derived from the person or programmer that created the list.

That it is a 15 element vector MIGHT be related to the fact that if you took only the upper or lower triangle of a 5x5 array, there would be exactly 15 elements. But that would be pure and wild conjectiure on my part, and I would still not know in which order those elements are taken. I cannot read the mind of the programmer who created this vector.