If \(D \gt 0\text{,}\) then
\begin{equation*}
T^2 - 4D \lt T^2.
\end{equation*}
Since we are considering the case \(T \gt 0\text{,}\) we have
\begin{equation*}
\sqrt{T^2 - 4D} \lt T
\end{equation*}
and the value of the second eigenvalue \((T - \sqrt{T^2 - 4D}\,)/2\) is postive. Therefore, any point in the first quadrant below the parabola corresponds to a system with two positive eigenvalues and must correspond to a nodal source.