A. V. Strelets

Any abstract Gram operator is consistent with some graph. For an arbitrary operator $B_\Gamma$ that is consistent with a graph $\Gamma$, the question arises as to when it is an abstract Gram operator, i.e., whether it is nonnegative. We study this question for certain types of graphs. The simplest case is a star graph. Next, we use the results obtained for star graphs to explore a more general case, where a graph $\Gamma$ can be treated as a collection of rooted trees, with their roots connected by additional edges into a connected subgraph $\Gamma_0$. The work shows that the question about the nonnegativity of an operator $B_\Gamma$ for such a graph can be reduced to the corresponding question for some operator that is consistent with the subgraph $\Gamma_0$.

We study systems of subspaces $H_1,\dots,H_N$ of a complex Hilbert space H that satisfy the following conditions: for every index $k > 1$, the set $\{\theta_{k,1},\ldots,\theta_{k,m_k}\}$ of angles $\theta_{k,i}\in(0,\pi/2)$ between $H_1$ and $H_k$ is fixed; all other pairs $H_k$, $H_j$ are orthogonal. The main tool in the study is a construction of a system of subspaces of a Hilbert space on the basis of its Gram operator (the G-construction).

For a Hilbert space $H$, we study configurations of its subspaces related to Coxeter graphs $\mathbb{G}_{s_1,s_2}$, $s_1,s_2\in\{4,5\}$, which are arbitrary trees such that one edge has type~$s_1$, another one has type~$s_2$ and the rest are of type~$3$. We prove that such irreducible configurations exist only in a finite dimensional $H$, where the dimension of $H$ does not exceed the number of vertices of the graph by more than twice. We give a description of all irreducible nonequivalent configurations; they are indexed with a continuous parameter. As an example, we study irreducible configurations related to a graph that consists of three vertices and two edges of type $s_1$ and $s_2$.