def sigmoid_function(x1,k,xo,a,c):
return (a/ (1+ np.exp(-k*(x1-xo))))+c
x_data=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]
y_data =[0.08965066,0.08990541,0.090073960.09013885,0.09021248,0.09038204,
0.09044601,0.09062396,0.09074469,0.09097