Besides the second summation starts and ends at n1/2, your code should be like this (although you should make sure your data is right, you set n1 = n2 = 3 and start the summations at n1/2 = 1.5, what is a non-sense):
n1 = 3;
n2 = 3;
g = 0; % initialization of summations
lim_inf_1= n1/2;
lim_sup_1 = n1*2;
lim_inf_2 = ? % your limits are....?
lim_sup_2 = ?
for i=lim_inf_1:lim_sup_2
for j=lim_inf_2:lim_sup_2
g = g + f(i,j);
end
end
