Approximately Strongly Regular Graphs

We give variants of the Krein bound and the absolute bound for graphs with a spectrum similar to that of a strongly regular graph. In particular, we investigate what we call approximately strongly regular graphs. We apply our results to extremal problems. Among other things, we show the following: (1) Caps in $\mathrm{PG}(n, q)$ for which the number of secants on exterior points does not vary too much, have size at most $O(q^{\frac34 n})$ (as $q \rightarrow \infty$ or as $n \rightarrow \infty$). (2) Optimally pseudorandom $K_m$-free graphs of order $v$ and degree $k$ for which the induced subgraph on the common neighborhood of a clique of size $i \leq m-3$ is similar to a strongly regular graph, have $k = O(v^{1 - \frac{1}{3m-2i-5}})$.

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