How to calculate a sum?

I have a sum S (the greek symbol) from p=0 to M-1, g=(Is/VT)*exp(x/VT); and I want: ans=1/M * sum(g*exp(-j*(2*pi/M)*p)). x is a 21x1 vector and i need an answer 21x21.

Please help!

Answer 1

[Walter Roberson]

g=(Is/VT)*exp(x/VT);
1/M * sum(g * exp(-j*(2*pi/M).*(0:M-1)))

However, if x is a vector, you need to clarify what shape it is (row or column) and need to clarify whether you want matrix multiplication for the "g*exp" part or what you want to do if x is a vector.

[P]

X=21x1 double array.And my ans needs to be a matrix with 21 values

[Walter Roberson]

g=(Is/VT)*exp(x/VT);

1/M * sum(g * exp(-j*(2*pi/M).*(0:M-1)),2)

[P]

So for my algorithm this ans needs to agree with another matrix y=21x21 because I want to do ans+y.How can i do that? your result are are 1x21 vector. thx a lot walter!

[P]

srry the result is 21x1 vector but still i can't sum this with a 21x21 matrix

[Walter Roberson]

If your x is 21x1 then what I proposed will produce a 21x1 output, not a 1 x 21. For example, size(sum((1:21).' * (0:4),2)) yields 21 1

Are you expecting a 21x21 matrix of different values, or are you wanting to copy the 21x1 matrix out to 21x21 ? If you want to copy it out to 21x21 then

repmat(1/M * sum(g * exp(-j*(2*pi/M).*(0:M-1)),2),1,21)

if you want all the rows to be the same, or

repmat((1/M * sum(g * exp(-j*(2*pi/M).*(0:M-1)),2)).',21,1)

if you want all the columns to be the same

[P]

I was expecting 21x 21 different values because M=21 points(-10...0...10) . I was expecting something like that:

a(0) a(-1) a(-2)....

a(1) a(0) a(-1)....

a(2) a(1) a(0).....