Thursday, 20 October 2016

determinant - Polynomial interpolation over the integers such that all coefficients are from the integers

If one is given t different points of a polynomial (all values are from the integers), is it then always possible to construct a polynomial of degree t such that it interpolates all points AND all coefficients are from the integers?




Second: What if some of the points correspond to derivatives? So can the Brikhoff interpolation problem with t points given be used to interpolate a polynomial of degree t such that all coefficients are from the integers?



Note that it is wanted that we are only given t points to interpolate a polynomial of degree t. This gives one degree of freedom. Otherwise it is easy to find a counterexample.



First question: Let x1,x2,,xtN0 such that all xi are distinct and ordered, i.e., 0x1<x2<x3<<xt. And let let y1,y2,,ytZ. Does there exist a polyonimial f(x)=a0+a1x+a2x2++atxt such that for all i it holds that f(xi)=yi and all ajZ.



Second question: Now assume that ci11,ci22,,cittZ, where ijN0 is just an indice (not the power). For these indices it holds that 0i0i1it<t and at least one ij>0.



Does there exist a polyonimial f(x)=a0+a1x+a2x2++atxt such that for all j it holds that fij(xj)=cijj and all ajZ, where fij(x) denotes the ij-th derivative of f(x).







I tried to solve the second question with Birkhoff interpolation. The Birkhoff interpolation can be used to reconstruct the function and also single coefficients: The interpolation of one coefficient is based on a matrix A which is determined by all xj and cijj. Then a coefficient ak1 is computed as det(Ak)/det(A) where Ak is obtained from A by replacing the k-th column of A with the cijj in lexicographic order. However, I'm not able to proof that det(Ak)/det(A)Z. Note that if we want to interpolate the polynomial of degree t with only t points/derivatives given, then we have to see the birkhoff interpolation problem as a problem where we are given t+1 points/derivates but we are allowed to modify one point (xz,cizz) arbitrarily.



The problem is also closely related to determinants, but I have very little knowledge in this area.



Until now, I couldn't construct a counterexample for it or proof it.







A proof, counterexample or any hints where to get additional information would be great! Or maybe someone knows something about the eigenvalues of the matrix of the Birkhoff interpolation?

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