I think this will work.
% % % % % CREATE MASK FOR MATRIX MULTIPLICATION % % % % %
% each column computes the summation for 1 element of 'fprod'
% get probnlem size (i.e., numel(x))
nx = numel(x);
% reduce n if it is needlessly large.
n = min(nx+1,n);
% build upper half of mask
mask = true(max(nx,n-1),n-1);
mask = tril(mask);
% crop edge of upper half if needed
if nx > n-1
mask = mask(1:end-(nx-n+1),:);
end
% add in lower half
mask = [mask;flipud(mask(1:end-1,:))];
% % % % % MAIN MATRIX MULTIPLY % % % % %
% compute fprod using the mask
fprod = x(:)*mask;
% sum fprod to get f
f = fprod*true(nx,1);
