想用三次样条法估计国债收益率曲线，B0=1+bt+ct*t+dt*t*t*t              (0<t<5)

                                                           B5=B0+(e-d)(t-5)*(t05)*(t-5)  (5<t<8)

                                                           B8=B5+(f-e)(t-8)*(t-8)*(t-8)    (8<t<20)

要对贴现函数进行广义最小二乘法估计，但是首先应该确认这个贴现函数的值吧，怎么确认贴现函数B的值呢?