Hakop Hakopian

Vahagn Bayramyan, Hakop Hakopian

In this paper we consider n-poised planar node sets, as well as more special ones, called $GC_n$-sets. For these sets all $n$-fundamental polynomials are products of n linear factors as it always takes place in the univariate case. A line ${\ell}$ is called $k$-node line for a node set $\mathcal X$ if it passes through exactly $k$ nodes. An $(n+1)$-node line is called maximal line. In 1982 M. Gasca and J. I. Maeztu conjectured that every $GC_n$-set possesses necessarily a maximal line. Till now the conjecture is confirmed to be true for $n \le 5$. It is well-known that any maximal line $M$ of $\mathcal X$ is used by each node in $\mathcal X\setminus M,$ meaning that it is a factor of the fundamental polynomial of each node. In this paper we prove, in particular, that if the Gasca-Maeztu conjecture is true then any $n$-node line of $GC_n$-set $\mathcal X$ is used either by exactly $\binom{n}{2}$ nodes or by exactly $\binom{n-1}{2}$ nodes. We prove also similar statements concerning $n$-node or $(n-1)$-node lines in more general $n$-poised sets. This is a new phenomenon in $n$-poised and $GC_n$ sets. At the end we present a conjecture concerning any $k$-node line.

Hakop Hakopian, Vahagn Vardanyan

An $n$-poised node set $\mathcal X$ in the plane is called $GC_n$ set if the (bivariate) fundamental polynomial of each node is a product of n linear factors. A line is called $k$-node line if it passes through exactly $k$-nodes of $\mathcal X.$ An $(n+1)$-node line is called maximal line. The well-known conjecture of M. Gasca and J. I. Maeztu states that every $GC_n$ set has a maximal line. Untill now the conjecture has been proved only for the cases $n \le 5.$ We say that a node uses a line if the line is a factor in the node's fundamental polynomial. It is a simple and well-known fact that any maximal line $M$ is used by all $\binom{n+1}{2}$ nodes in $\mathcal X\setminus M.$ Here we consider the main result of the paper - V. Bayramyan, H. Hakopian, On a new property of n-poised and $GC_n$ sets, Adv Comput Math, 43, (2017) 607-626, stating that any $n$-node line of $GC_n$ set is used either by exactly $\binom{n}{2}$ nodes or by exactly $\binom{n-1}{2}$ nodes, provided that the Gasca-Maeztu conjecture is true. In this paper we show that this result is not correct in the case $n=3.$ Namely, we bring an example of a $GC_3$ set and a $3$-node line there which is not used at all. Fortunately, then we were able to establish that this is the only possible counterexample, i.e., the above mentioned result is true for all $n\ge 1, n\neq 3.$ We also characterize the exclusive case $n=3$ and present some new results on the maximal lines and the usage of $n$-node lines in $GC_n$ sets.

Hayk Avdalyan, Hakop Hakopian

There are several examples of spaces of univariate functions for which we have a characterization of all sets of knots which are poised for the interpolation problem. For the standard spaces of univariate polynomials, or spline functions the mentioned results are well-known. In contrast with this there are no such results in the bivariate case. As an exception one may consider only the Pascal classic theorem, in the interpolation theory interpretation. In this paper we consider a space of bivariate piecewise linear functions, for which we can readily find out whether the given node set is poised or not. The main tool we use for this purpose is the reduction by a basic subproblem, introduced in this paper.