MATLAB: Operation of specific elements of a matrix

MATLABmatrixmatrix arraymatrix manipulation

I have a matrix of dimensions 3000×3.

I need to compute statistics for the elements (1,1),(4,1),(7,1), (10,1),…,(3000,1)

how can I code this statistic i.e mean? It is basically the mean of the elements of the first colum, which are in rows 1,4,7,10…3000 i.e i+3?

mean(matrix(1:3:end,1))

a = a(1:3);  %glass half full

a(4) = []; %glass half empty

[m k] = size(A);
i1 = (1:m)';
r = 1:k;
Mi = [2; 4; 1];
Mr = 1;
out1 = bsxfun(@rdivide,i1*r.*q.^2,Mi)*Mr;
out = sum(out1(:)) - 1/log(n);

ADD

out2 = max(out1)  - 1/log(n);