Erdős–Ko–Rado theorem

The original proof of 1961 used induction on n. In 1972, Gyula O. H. Katona gave the following short proof using double counting.

Suppose we have some such family of subsets A. Arrange the elements of {1, 2, ..., n} in any cyclic order, and consider the sets from A that form intervals of length r within this cyclic order. For example if n = 8 and r = 3, we could arrange the numbers {1, 2, ..., 8} into the cyclic order (3,1,5,4,2,7,6,8), which has eight intervals:

(3,1,5), (1,5,4), (5,4,2), (4,2,7), (2,7,6), (7,6,8), (6,8,3), and (8,3,1).

However, it is not possible for all of the intervals of the cyclic order to belong to A, because some pairs of them are disjoint. Katona's key observation is that at most r of the intervals for a single cyclic order may belong to A. To see this, note that if (a₁, a₂, ..., a_r) is one of these intervals in A, then every other interval of the same cyclic order that belongs to A separates a_i and a_i+1 for some i (that is, it contains precisely one of these two elements). The two intervals that separate these elements are disjoint, so at most one of them can belong to A. Thus, the number of intervals in A is one plus the number of separated pairs, which is at most (r - 1).

Based on this idea, we may count the number of pairs (S,C), where S is a set in A and C is a cyclic order for which S is an interval, in two ways. First, for each set S one may generate C by choosing one of r! permutations of S and (n − r)! permutations of the remaining elements, showing that the number of pairs is |A|r!(n − r)!. And second, there are (n − 1)! cyclic orders, each of which has at most r intervals of A, so the number of pairs is at most r(n − 1)!. Combining these two counts gives the inequality

${\displaystyle |A|r!(n-r)!\leq r(n-1)!}$

and dividing both sides by r!(n − r)! gives the result

${\displaystyle |A|\leq {\frac {r(n-1)!}{r!(n-r)!}}={n-1 \choose r-1}.}$

Two constructions for an intersecting family of r-sets: fix one element and choose the remaining elements in all possible ways, or (when n = 2r) exclude one element and choose all subsets of the remaining elements. Here n = 4 and r = 2.

There are two different and straightforward constructions for an intersecting family of r-element sets achieving the Erdős–Ko–Rado bound on cardinality. First, choose any fixed element x, and let A consist of all r-subsets of ${\displaystyle \{1,2,...,n\}}$ that include x. For instance, if n = 4, r = 2, and x = 1, this produces the family of three 2-sets

{1,2}, {1,3}, {1,4}.

Any two sets in this family intersect, because they both include x. Second, when n = 2r and with x as above, let A consist of all r-subsets of ${\displaystyle \{1,2,...,n\}}$ that do not include x. For the same parameters as above, this produces the family

{2,3}, {2,4}, {3,4}.

Any two sets in this family have a total of 2r = n elements among them, chosen from the n − 1 elements that are unequal to x, so by the pigeonhole principle they must have an element in common.

When n > 2r, families of the first type (variously known as sunflowers, stars, dictatorships, centred families, principal families) are the unique maximum families. Friedgut (2008) proved that in this case, a family which is almost of maximum size has an element which is common to almost all of its sets. This property is known as stability.

The seven points and seven lines (one drawn as a circle) of the Fano plane form a maximal intersecting family.

An intersecting family of r-element sets may be maximal, in that no further set can be added without destroying the intersection property, but not of maximum size. An example with n = 7 and r = 3 is the set of 7 lines of the Fano plane, much less than the Erdős–Ko–Rado bound of 15.

There is a q-analog of the Erdős–Ko–Rado theorem for intersecting families of subspaces over finite fields. Frankl & Wilson (1986)

If ${\displaystyle S}$ is an intersecting family of ${\displaystyle k}$-dimensional subspaces of an ${\displaystyle n}$-dimensional vector space over a finite field of order ${\displaystyle q}$, and ${\displaystyle n\geq 2k}$, then

${\displaystyle \vert S\vert \leq {\binom {n-1}{k-1}}_{q}.}$

The Erdős–Ko–Rado theorem gives a bound on the maximum size of an independent set in Kneser graphs contained in Johnson schemes.

Similarly, the analog of the Erdős–Ko–Rado theorem for intersecting families of subspaces over finite fields gives a bound on the maximum size of an independent set in q-Kneser graphs contained in Grassmann schemes.

• Bollobás, B.; Leader, I. (1997), "An Erdős-Ko-Rado theorem for signed sets", Computers and Mathematics with Applications, 34 (11): 9–13, doi:10.1016/S0898-1221(97)00215-0, MR 1486880
• Erdős, P. (1987), "My joint work with Richard Rado", in Whitehead, C. (ed.), Surveys in combinatorics, 1987: Invited Papers for the Eleventh British Combinatorial Conference (PDF), London Mathematical Society Lecture Note Series, 123, Cambridge University Press, pp. 53–80, ISBN 978-0-521-34805-8.
• Erdős, P.; Ko, C.; Rado, R. (1961), "Intersection theorems for systems of finite sets" (PDF), Quarterly Journal of Mathematics. Oxford. Second Series, 12: 313–320, doi:10.1093/qmath/12.1.313.
• Frankl, P.; Wilson, R. M. (1986), "The Erdős-Ko-Rado theorem for vector spaces", Journal of Combinatorial Theory, Series A, 43: 228–236, doi:10.1016/0097-3165(86)90063-4.
• Friedgut, Ehud (2008), "On the measure of intersecting families, uniqueness and stability" (PDF), Combinatorica, 28 (5): 503–528, doi:10.1007/s00493-008-2318-9
• Katona, G. O. H. (1972), "A simple proof of the Erdös-Chao Ko-Rado theorem", Journal of Combinatorial Theory, Series B, 13 (2): 183–184, doi:10.1016/0095-8956(72)90054-8.

• Godsil, Christopher; Karen, Meagher (2015), Erdős–Ko–Rado Theorems: Algebraic Approaches, Cambridge Studies in Advanced Mathematics, Cambridge University Press, ISBN 9781107128446.