$$
dr(t) = k(\theta-r(t))dt+\sigma\sqrt{r(t)}dW(t)\\
SpotRates(t) -\frac{ A(t)}{t}+\frac{B\left(t\right)}{t} \times
r\left(0\right)$$
$$ SpotRates(t)= \\
\beta_{0}+\beta_{1}\left({\frac{1-e^{-\frac{t}{\tau}}}{\frac{t}{\tau}}} \right)+
\beta_{2}\left({\frac{1-e^{-\frac{t}{\tau}}}{\frac{t}{\tau}}}-e^{-\frac{t}{\tau}}\right)
$$
$$ SpotRates(t)= \\
\beta_{0}+\beta_{1}\left({\frac{1-e^{-\frac{t}{\tau_{1}}}}{\frac{t}{\tau_{1}}}}
\right)+
\beta_{2}\left({\frac{1-e^{-\frac{t}{\tau_{1}}}}{\frac{t}{\tau_{1}}}}-e^
{-\frac{t}{\tau_{1}}}\right)+ \\
\beta_{3}\left({\frac{1-e^{-\frac{t}{\tau_{2}}}}{\frac{t}{\tau_{2}}}}-e^
{-\frac{t}{\tau_{2}}}\right)$$
\begin{eqnarray*}
dr(t) &=& k(\theta-r(t))dt+\sigma dW (t)\\
SpotRates(t) &=&
-\frac{ A(t)}{t} +\frac{B\left(t\right)}{t} \times r\left(0\right), \\
B\left(t\right) &=& \frac{1-e^{-\kappa (t)}}{\kappa}, \\
A\left(t\right) &=& \left(B\left(t\right)-(t)\right)\left(\theta-\frac{\sigma^2}
{2\kappa^2}\right)-\frac{\sigma^2B\left(t\right)^2}{4\kappa} \end{eqnarray*}