# Intersecting Families

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1 Intersecting Families Extremal Combinatorics Philipp Zumstein 1 The Erds-Ko-Rado theorem 2 Projective planes Maximal intersecting families 4 Helly-type result A familiy of sets is intersecting if any two of its sets have a nonempty Let be an intersecting family of subsets of {1,...,n} = [n]. Question: How large can such a family be? Take all subsets containing a fixed element. This is an intersecting family with Can we find larger intersecting families? No! A set an its complement cannot both be members of So we get: A familiy of sets is intersecting if any two of its sets have a nonempty Let be an intersecting family of k-element subsets of {1,...,n} = [n]. Question: How large can such a family be? Trivial upper bound: First case: n < 2k: - Every pair of k-element subsets of [n] has a non-empty - So we could choose as the set of all k-element subsets of [n] - So the trivial upper bound is sharp. Second case: n 2k Take all the k-element subsets containing a fixed element. Examples: n = 5, k = 2, fix the element 1 {1,2}, {1,}, {1,4}, {1,5} n = 5, k =, fix the element 1 {1,2,}, {1,2,4}, {1,2,5}, {1,,4}, {1,,5}, {1,4,5} Theorem: (Erds-Ko-Rado, 1961) If 2k n then every intersecting family of k-element subsets of an n-element set has at most members. This is an intersecting family with Can we find larger intersecting families? 1

2 Proof: (due to G.O.H. Katona, 1972) W.l.o.g. we can assume X = {0,1,...,n-1}. For s X, define Proof: Assume B 0 B k-2 k-1... n-1 B -k+1 B 1 B 2 where the addition is modulo n. B -k+2 B -k+... B... B -2 B -1 B k-2 B k-1 There are 2k-2 sets that intersect with B 0. These sets can partioned into k-1 pairs of disjoint sets B i, B i+k, where (k-1) i -1. Since can contain at most one set of each pair the assertion of the claim follows. Proof: W.l.o.g. we can assume X = {0,1,...,n-1}. For s X, define where the addition is modulo n. L := number of pairs (,s), where is a permutation of X and s is a point of X, such that the set (B s ) = {(s), (s+1),..., (s+k-1)} belongs to Double counting: Summary A familiy of sets is intersecting if any two of its sets have a non-empty Let be a family of subset of {1,...,n} = [n]. Question: How large can such a family be? (Maximum) Let be a family of k-element subsets of {1,...,n} = [n]. n < 2k: n 2k: Together: Erds-Ko-Rado Projective planes 1. Each pair of points determines a unique line. 2. Each two lines intersect in exactly one point.. Any point lies on q+1 lines. 4. Every line has q+1 points. 5. There are q 2 +q+1 points. 6. There are q 2 +q+1 lines. Each pair of points determines a unique line. A projective plane of order q has the following Proof: (a) Take a point x There are q(q+1) other points Each line through x contain q further points Two such lines don t overlap (apart from x) Each point lies on a line through x So, there are exactly q+1 lines through x. (iii) (ii) 2

3 Each pair of points determines a unique line. A projective plane of order q has the following Proof: (b) Counting the pairs (x,l) with x L in two ways: Each pair of points determines a unique line. A projective plane of order q has the following Proof: (c) Let L 1 and L 2 be lines, and x a point of L 1 (and not L 2 ). Then the q+1 points from L 2 are joined to x by different lines. x lies on exactly q+1 lines. So one of this lines has to be L 1. But then, L 1 and L 2 intersect in exactly one point. q = 1: Example and Duality Points: X = { 0,1,2 } Lines: = { {0,1}, {1,2}, {2,0} } The construction And the vectorspace K We define our points as 1-dimensional subspaces of K, i.e. 2 c b 0 a 1 projective plane of order 1 c a b dual projective plane for (x 0,x 1,x 2 ) V := K - (0,0,0). (Note: If x 0 = x 1 = x 2 = 0 then this is a 0-dimensional subspace. So we don t allow this case.) Such a point is a set of q-1 vectors from V. There are (q -1) / (q-1) = q 2 +q+1 such points. This shows condition (iii). For (x 0,x 1,x 2 ) V := K - (0,0,0) we define the points For (x 0,x 1,x 2 ) V := K - (0,0,0) we define the points The line L(a 0,a 1,a 2 ), where (a 0,a 1,a 2 ) V, is defined to be the set of all those points [x 0,x 1,x 2 ] for which a 0 x 0 + a 1 x 1 + a 2 x 2 = 0. Two triples (x 0,x 1,x 2 ) and (cx 0,cx 1,cx 2 ) either both satisfy this equation or none does. How many points does such a line have? Because (a 0,a 1,a 2 ) V, this vector has at least one nonzero component; say a 0 0. Chose x 1 and x 2 arbitrary not both 0 and (because K is a field) we can uniquely determine x 0. So we get q 2-1 solutions (x 0,x 1,x 2 ) V. These are q+1 points. This shows (ii). The line L(a 0,a 1,a 2 ), where (a 0,a 1,a 2 ) V, is defined to be the set of all those points [x 0,x 1,x 2 ] for which a 0 x 0 + a 1 x 1 + a 2 x 2 = 0. Let [x 0,x 1,x 2 ] and [y 0,y 1,y 2 ] be two distinct points. How many lines contain both these points? For each such line L(a 0,a 1,a 2 ) Since the matrix a 0 x 0 + a 1 x 1 + a 2 x 2 = 0 a 0 y 0 + a 1 y 1 + a 2 y 2 = 0 has rank 2 (the rows are linearly independent), the solution-space is 1- dimensional, i.e. one point. This shows.

4 Example: Fano Plane 100 q = 2 Projective plane with 7 points and points on a line. K = GF(2), V = K 000 = { 001, 010, 011, 100, 101, 110, 111 } These are also the points. Lines: v V equation: line: 001 x 2 = 0 L(001) = { 010, 100, 110 } 010 x 1 = 0 L(010) = { 001, 100, 101 } 011 x 1 +x 2 = 0 L(011) = { 011, 100, 111 } 100 x 0 = 0 L(100) = { 010, 001, 011 } 101 x 0 +x 2 = 0 L(101) = { 010, 101, 111 } 110 x 0 +x 1 = 0 L(110) = { 001, 110, 111 } 111 x 0 +x 1 +x 2 = 0 L(111) = { 011, 101, 110 } Bruck-Chowla-Ryser Theorem: If a projective plane of order n exists, where n is congruent 1 or 2 modulo 4, then n is the sum of two squares of integers. There is no projective plane of order 6 or 14. What about 10? Is there a projective plane of order 10? 1988: There is no projective plane of order 10 Open Question: Is there a projective plane of order 12? Summary 1. Each pair of points determines a unique line. 2. Each two lines intersect in exactly one point.. Any point lies on q+1 lines. 4. Every line has q+1 points. 5. There are q 2 +q+1 points. 6. There are q 2 +q+1 lines. If q = p r is a power of a prime number, then there exist a projective plane of order q. Maximal intersecting families Let be a k-uniform family of sets of some n-element set. Maximal intersecting families Let be a k-uniform family of sets of some n-element set. is maximal intersecting if is intersecting; (ii) the addition of any new k-element set to destroys this property. is maximal intersecting if is intersecting; (ii) the addition of any new k-element set to destroys this property. Examples: n = 8, k = 2 Example: n = 7, k = = { {1,2}, {1,}, {1,4}, {1,5}, {1,6}, {1,7}, {1,8} } Can we get a maximal intersecting family with fewer subsets? = { {1,2}, {2,}, {,1} } Yes! = { {1,2,}, {1,4,5}, {1,6,7}, {2,4,6}, {2,5,7}, {,4,7}, {,5,6} } {4,5,6}, {2,,6}, {2,,4}, {1,,5}, {1,,4}, {1,2,5}, {1,2,4} {4,5,7}, {2,,7}, {2,,5}, {1,,7}, {1,,6}, {1,2,6}, {1,2,7} {4,6,7}, {2,6,7}, {2,4,5}, {1,5,7}, {1,4,6}, {1,5,6}, {1,4,7} {5,6,7}, {,6,7}, {,4,5}, {,5,7}, {,4,6},{2,5,6}, {2,4,7} 1 2 Is this family maximal intersecting? Yes! 4

5 Example: n = 7, k = = { {1,2,}, {1,4,5}, {1,6,7}, {2,4,6}, {2,5,7}, {,4,7}, {,5,6} } One case: n < 2k: The only maximal intersecting family is the family of all k-element subsets. Another case: n k 2 -k+1: Consider the family of lines in a projective plane of order k-1. There are k 2 -k+1 lines and each line is a k-element subset of an n- element set of points. Any two lines intersect in precisely one point; so is intersecting. Claim: is maximal intersecting Proof: (indirect) Let E be a k-element set which intersects all the lines. Assume E is not a line (i.e. E is not a member of ). Take two points x y of E. Let L be the line through x and y. Take a point z L - E. z belongs to k lines, and each of them intersect E. The intersection with L contains at least 2 elements x,y. E > k. Proof: N = number of pairs (F,E) where F disjoint from F (and hence, E ) and E is a k-element subset Theorem (Füredi, 1980): Double counting! Let be a maximal intersecting family of k-element sets of an n-element set. Then Together (ii) In particular: for (ii) The (stronger) assumption leads to E. Helly, 192: A Helly-type result If n k+1 convex sets in have the property that any k+1 of them have a nonempty intersection, then there is a point common to all of them. Special case: k = 1: convex sets in = intervals We take n 2 such intervals with the property that any two of them have a nonempty We claim that there is a point common to all of them. A Helly-type result Let be a family and k be the minimum size of its member. If any k+1 members of intersect (i.e., share a common point) then all of them do. Proof: Assume the opposite, that the intersection of all sets in is empty. Take a set A = {x 1,...,x k } x i is not in every set of of minimum size. So, there is a set B i such that x i B i for every i = 1,..., k We get 5

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