单输出的神经元网络拟合有噪声干扰的函数：y=x1/(1+x1) + x2/(1+x2) + x3/(1+x3) + x4/(1+x4)-single output neuron network fitting a noise function : y = x/(1 x1) x2/(1 x2) x3/(1 x3) x/(1 x4)