### Interesting problems that I cannot solve

A set of

*n*points in general position in**R**^{d}determines simplices. It is always possible to choose a point in**R**^{d}, but not necessarily in the set, which lies in a positive proportion of the simplices. How large is this positive proportion?Denote by

*c*(*d*) the supremum of all*c*, so that it is possible to choose point in proportion*c*of all the simplices. Then*c*(*1*)=*1*/*2*,*c*(*2*)=*2*/*9*and*2d*/(*d*+*1*)(*d*+*1*)!≤*c*(*d*)≤(*d*+*1*)!(*d*+*1*)^{-d-1}.For a graph

*G*let*G*_{p}denote the subgraph of*G*, in which each edge of*G*is in*G*_{p}with probability*p*independently at random. For any graph*H*let χ(*H*) denote the chromatic number of*H*. Is there a constant*c*>0 so that E(χ(*G*_{1/2}))>*c*χ(*G*)/`log`χ(*G*)?Since

*G*_{1/2}and its complement in*G*have the same distribution, E(χ(*G*_{1/2}))≥χ(G)^{1/2}. Also, the inequality E(χ(*G*_{1/2}))>*c*χ(*G*)/`log`*n*holds, where*n*is the number of vertices in*G*.For a finite set of integers

*A*define*A*+*A*={*a*+*a*' :*a*,*a*'∈*A*} and*A*+2⋅*A*={*a*+2*a*' :*a*,*a*'∈*A*}. Is there an*ε*>0 such that |*A*+2⋅*A*|/|*A*|≤(|*A*+*A*|/|*A*|)^{3-ε}for all finite sets*A*?The inclusion

*A*+2⋅*A*⊆*A*+*A*+*A*and Plünnecke's inequality together imply that |*A*+2⋅*A*|/|*A*|≤(|*A*+*A*|/|*A*|)^{3}. The analogous question with 2 replaced by any sufficiently large integer has affirmative answer.For a polynomial

*f*in one real variable let*T*(*f*) be the number of non-zero terms in*f*. Is there an*ε*>0 such that*T*(*f*^{2})≥*T*(*f*)^{ε}?There are polynomials with arbitrarily many terms for which

*T*(*f*^{2})≤*T*(*f*)^{99/100}. It is known that*T*(*f*^{2})≥(1/100)`log`*T*(*f*).For a set

*A*⊆**Z**^{d}and linear maps*L*,…,_{1}*L*:_{k}**Z**^{d}→**Z**^{d}define*L*+…+_{1}A*L*={_{k}A*L*+…+_{1}a_{1}*L*:_{k}a_{k}*a*,…,_{1}*a*∈_{k}*A*}. Suppose*L*_{1}**Z**^{d}+…+*L*_{k}**Z**^{d}=**Z**^{d}and*L*,…,_{1}*L*have no common invariant subspace. Does it follow that |_{k}*L*+…+_{1}A*L*|≥[|det_{k}A*L*|_{1}^{1/d}+…+|det*L*|_{k}^{1/d}]^{d}|*A*|(*1*+*o*(*1*)) for every finite set*A*?The inequality is true if

*d*=*1*. If*A*were a closed subset of**R**^{d}, then the analogous inequality for the Lebesgue measure would hold by Brunn–Minkowski theorem for every*d*.For an

*r*-uniform hypergraph*H*and a (*r*-*2*)-element set*S*the link of*H*over*S*is the graph {(*u*,*v*) : {*u*,*v*}∪*S*∈*H*}. Let*d*(*r*) be the maximum density of an*r*-uniform hypergraph such that no link contains a copy of*K*_{4}. Is`lim`*d*(*r*)=*0*as*r*→∞?Averaging implies that

*c*(*2*)≥*c*(*3*)≥*c*(*4*)≥…, and hence the limit exists. The limit is at most*1*/√*3*. For the analogous question for*K*_{3}the limit is zero, with the rate of decay between`log`*r*/*r*and^{2}*1*/*r*. The analogous question for*C*has negative answer._{4}For a graph

*G*, the*n*'th tensor power*G*^{⊗n}is the graph with the vertex set*V*(*G*)^{n}in which the two vertices (*x*,…,_{1}*x*) and (_{n}*y*,…,_{1}*y*) are_{n}*not*adjacent if there is an*i*for which*x*and_{i}*y*are not adjacent in_{i}*G*. Let*C*denote the_{5}*5*-cycle graph. Let*f*(*n*) be the independence number of*C*_{5}^{⊗n}. What is the`lim`*f*(*2n*+*1*)/*5*?^{n}Since

*f*(*n*+*2*)≥*f*(*2*)*f*(*n*)=5*f*(*n*) the limit exists and is at least*2*. Lovász's theta function gives*f*(*n*)≤*5*^{n/2}, and thus the limit is at most √*5*. Furthermore*f*(*3*)=*10*.Consider

*r*pairs of points (*x*,_{1}*y*),…,(_{1}*x*,_{r}*y*) on a sphere_{r}**S**^{n}and*r*continuous functions*f*,…,_{1}*f*:_{r}**S**^{n}→**R**. If*r*is fixed and*n*is large, is there necessarily a rotation*σ*∈SO(*n*+*1*) such that*f*(_{i}*σx*)=_{i}*f*(_{i}*σy*) for all_{i}*i*=*1*,…,*r*?This true for

*r*=*1*and for*r*=*2*. Then*n*=*1*and*n*=*2*respectively. However,*n*=*r*does not suffice in general.Is there, for every three-element set

*L*⊂**F**_{p}and every*n*, an*n*-by-*n*matrix of rank*O*(*n*^{1/3}) that has zeros on the diagonal, and elements of*L*everywhere else?The simplest open case is {

*1*,*2*,*4*} in**F**_{13}. The*n*^{1/3}, if attainable, is sharp.What is the maximum number of

*n*-bit words no two of which have a common subsequence of length*0.501*·*n*?The number is known to be between

`log`*n*-*O*(*1*) and*2*^{O(n)}.

### ...but someone else can!

The height of a rational number

*p*/*q*, where`gcd`(*p*,*q*)=1, is defined to be*H*(*p*/*q*)=|*p*|+|*q*|. Suppose*f*is a real-analytic function on some open interval, which takes rational numbers to rational numbers. Let*k*be a positive integer. Does*H*(*f*(*x*))≤*H*(*x*)^{k}for all rational*x*imply that*f*is a rational function with rational coefficients?I could show this to be true if one assumes that the function

*f*at some point of the domain has Taylor expansion with the first*2k*+*1*coefficients rational. By theorem 4 in the paper “Rational points near curves and small nonzero |x^3-y^2| via lattice reduction” by Noam Elkies it is always true.Let

*P*be a set of*n*points in**R**^{2}in general position and*L*be the set of lines determined by*P*. Is it possible that one can choose a collection of*O*(*n*) points disjoint from*P*such that every line in*L*contains at least one of the points in the collection?A linear lower bound on the size of the collection follows from the fact that no point in the collection can be contained in more than

*n*/2 of the lines. A superlinear lower bound can be deduced from Freiman's theorem under the stronger assumption that the collection contains the midpoint of each of the line segments. The answer to the question is negative, as shown in “Blocking visibility for points in general position” by Jiří Matoušek, which was pointed to me by David Wood.Let

*G*=(*V*,*E*) be a finite graph, and suppose that to each edge*e*∊*E*a function*f*_{e}, from a finite set*X*to a finite set*A*, is associated. Let*N*be the number of functions*g*:*V*→*X*satisfying*f*_{xy}(*g*(x))=*f*_{xy}(*g*(y)) for every edge*xy*. If*G*is the triangle, is there an*ε*>0 such that*N*≥|*X*|^{3}/|*A*|^{4-ε}?Imre Ruzsa showed that such an

*ε*does not exist. Let*X*be a subset of {1,...,*n*}^{2}of size |*X*|=*n*^{2-o(1)}not containing a corner of the form {(*x*,*y*),(*x*+*a*,*y*),(*x*,*y*+*a*)}. Its existence follows from Behrend's construction. Let*A*={1,...,2*n*} and put*f*_{12}(*x*,*y*)=*x*,*f*_{13}(*x*,*y*)=*y*,*f*_{23}(*x*,*y*)=*x+y*. If*ε*>0, for large*n*we reach a contradiction. Though*ε*=0, it is possible to prove that*w*(|*A*|)*N*≥|*X*|^{3}/|*A*|^{4}for some function*w*tending to infinity. If*G*is a tree, then*N*≥|*X*|^{|V|}/|*A*|^{|E|}.Let

*f*(*s*)=*a*_{1}+*a*_{2}2^{-s}+*a*_{3}3^{-s}+… be a Dirichlet series whose coefficients satisfy |*a*_{n}|≤*n*^{C}. Extend*f*meromorphically as far as possible. Is the domain of the meromorphic continuation a halfplane?The question was answered in negative by David Speyer

A

*k*-uniform hypergraph is quasirandom if every linearly-sized set*U*spans (*c*+*o*(*1*))|*U*|^k edges. For*k*≥*3*, give an*explicit*construction of a*k*-uniform hypergraph on*n*-vertices with*n*^{k/2}edges.It turned out this was answered by Alon–Feige–Wigderson–Zuckerman: one can take a regular graph

*G*with small second eigenvalue, and defined the hypergraph whose edges are the*k*-walks. The motivation for the question was that the usual method to show quasirandomness of explicit hypergraphs is via repeated applications of the Cauchy–Schwarz inequality. This method could not succeed with fewer than*n*^{k/2+ε}edges.