You are provided the following discount function data. Fit it to a cubic exponential system with knot points every half year until and including t = 4.5 years. 

t 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 d(t) 0.97092 0.95886 0.94881 0.94160 0.93464 0.92902 0.92497 0.91828 0.91395 0.90934 0.90569 0.90124 0.89710 0.89220 0.89058 0.88761 0.88480 0.88296 0.87922 0.87592