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J. Koolen posed the problem of studying distance-regular graphs in which neighborhoods of vertices are strongly regular graphs with the second eigenvalue ≤ t for the given positive integer t. This problem is reduced to the des­cription of distance-regular graphs in which neighborhoods of vertices are strongly regular graphs with the non-principal eigenvalue t for t = 1, 2,… In the paper “Distance regular graphs in which local subgraphs are strongly regular graphs with the second eigenvalue at most 3”, A.A. Makhnev and D.V. Paduchikh found the arrays of intersections of distance-regular graphs in which neighborhoods of vertices are strongly regular graphs with the second eigenvalue t such as 2 < t ≤ 3. The graphs with intersection arrays {125,96,1;1,48,125}, {176,150,1;1,25,176}, and {256,204,1;1,51,256} remain unexplored. In this paper, we have found the possible orders and the structures of subgraphs of the fixed points of automorphisms of a distance-regular graph with the intersection array {125,96,1;1,48,125}. It has been proved that the neighborhoods of the vertices of this graph are pseudogeometric graphs for GQ(4,6). Composition factors of the automorphism group of a distance-regular graph with the intersection array {125,96,1;1,48,125} have been determined.

We study a problem concerning the estimation of the Hausdorff deviation of convex polygons in $\mathbb R^2$ from their geometric difference with circles of sufficiently small radius. Problems with such a subject, in which not only convex polygons but also convex compacts in the Euclidean space $\mathbb R^n$ are considered, arise in various fields of mathematics and, in particular, in the theory of differential games, control theory, convex analysis. Estimates of Hausdorff deviations of convex compact sets in $\mathbb R^n$ in their geometric difference with closed balls in $\mathbb R^n$ are presented in the works of L.S. Pontryagin, his staff and colleagues. These estimates are very important in deriving an estimate for the mismatch of the alternating Pontryagin’s integral in linear differential games of pursuit and alternating sums. Similar estimates turn out to be useful in deriving an estimate for the mismatch of the attainability sets of nonlinear control systems in $\mathbb R^n$ and the sets approximating them. The paper considers a specific convex heptagon in $\mathbb R^2$. To study the geometry of this heptagon, we introduce the concept of a wedge in $\mathbb R^2$. On the basis of this notion, we obtain an upper bound for the Hausdorff deviation of a heptagon from its geometric difference with the disc in $\mathbb R^2$ of sufficiently small radius.