tony21248772

Results 2 issues of tony21248772

![image](https://github.com/lballabio/QuantLib/assets/24760275/e5e4fbc3-447b-47a8-84d2-01fe3693e1ed)

Hi, I have a question on the setup of the tridiagonal system in the cubic interpolation (under the choice 'Spline' for derivative approximation. The tridiagonal system looks a bit different...