# Modified Hermite interpolation

I asked similar questions here and here, but I tried to formulate this
one in a sharper way.

Is anyone aware of some literature on polynomial interpolation where we sample the function and its derivatives, but perhaps we sample the derivative at a point where we have not sampled the function?

For instance, given a function $f$, suppose we choose a polynomial $p$ of degree at most 3 where
$$p(a_1)=f(a_1)\\p(a_2)=f(a_2)\\p'(a_3)=f'(a_3)\\p'(a_4)=f'(a_4).$$

Finding such a polynomial $\alpha_3 x^3 + \alpha_2 x^2 + \alpha_1 x + \alpha_0$ is equivalent to solving
$$\left[ \begin{matrix} 1&a_1 & a_1^2 & a_1^3\\ 1&a_2 & a_2^2 & a_2^3\\ 0&1 & 2a_3 & 3a_3^2\\ 0&1 & 2a_4 & 3a_4^2 \end{matrix} \right] \left[ \begin{matrix} \alpha_0 \\ \alpha_1 \\ \alpha_2 \\ \alpha_3 \end{matrix} \right] = \left[ \begin{matrix} f(a_1) \\ f(a_2) \\ f'(a_3) \\ f'(a_4) \end{matrix} \right].$$

I claim that the determinant of that matrix is $\frac{\partial^2}{\partial x_3\partial x_4}\left( \prod_{i<j}(x_i-x_j) \right) \left. \right|_{x_i=a_i\,\,\forall i}$. Therefore, $\frac{\partial^2}{\partial x_3\partial x_4}\left( \prod_{i<j}(x_i-x_j) \right) \left. \right|_{x_i=a_i\,\,\forall i}\neq 0$ is a necessary and sufficient condition for such a polynomial to exist uniquely.

In particular, I am interested in estimating the error of such a polynomial interpolation, since I don’t see how to modify the normal Hermite estimation.

#### Solutions Collecting From Web of "Modified Hermite interpolation"

If $V(x_1, \ldots, x_n)$ is the Vandermonde matrix whose determinant is
$$\det V(x_1, \ldots, x_n), the fact that \det V(x_1,\ldots,x_n) is linear as a function of the i’th row tells you that the partial derivative of the determinant w.r.t. x_i is the determinant of the matrix with row i replaced by its derivative w.r.t. x_i. If you prefer, you could also get this from the Laplace expansion (aka minor expansion) along the i’th row:$$ \det V = \sum_k (-1)^{i+k} v_{ik} M_{ik} $$where the M_{ik} don’t depend on x_i, so$$ \dfrac{\partial}{\partial x_i} \det V = \sum_{k} (-1)^{i+k} \dfrac{\partial v_{ik}}{\partial x_i} M_{ik} 
which is the determinant of the matrix obtained from $V$ by replacing
each entry of row $i$ by its partial derivative wrt $x_i$.
Estimate the difference between $f$ and $p$ interpolating $f$
There is a unique polynomial interpolating $f$ and its derivatives