function act = activation_function(fu);

act = fu;
for i = 1:size(fu,1)
   for j = 1:size(fu,2)
      u = fu(i,j);
      if (u<0.0)
         act(i,j) = (1./(1.+exp(-u)));
      else
         act(i,j) = 0.5 * sqrt( 1.0 + u );
      end
   end
end