We study exak(n,F), the largest number of edges in an n-vertex graph that contains exactly k copies of a given subgraph F. The case k=0 is the Turán number ex(n,F) that is among the most studied parameters in extremal graph theory. We show that for any F and k, exak(n,F)=(1+o(1))ex(n,F) and determine the exact values of exak(n,K3) and exa1(n,Kr) for n large enough. We also explore a connection to the following well-known problem in search theory. We are given a graph of order n that consists of an unknown copy of F and some isolated vertices. We can ask pairs of vertices as queries, and the answer tells us whether there is an edge between those vertices. Our goal is to describe the graph using as few queries as possible. Aigner and Triesch in 1990 showed that the number of queries needed is at least n2−exa1(n,F). Among other results we show that the number of queries that were answered NO is at least n2−exa1(n,F).

Suppose that the vertices of a graph G are colored with two colors in an unknown way. The color that occurs on more than half of the vertices is called the majority color (if it exists), and any vertex of this color is called a majority vertex. We study the problem of finding a majority vertex (or show that none exists), if we can query edges to learn whether their endpoints have the same or different colors. Denote the least number of queries needed in the worst case by m(G). It was shown by Saks and Werman that m(Kn)=n−b(n), where b(n) is the number of 1’s in the binary representation of n. In this paper we initiate the study of the problem for general graphs. The obvious bounds for a connected graph G on n vertices are n−b(n)≤m(G)≤n−1. We show that for any tree T on an even number of vertices we have m(T)=n−1, and that for any tree T on an odd number of vertices, we have n−65≤m(T)≤n−2. Our proof uses results about the weighted version of the problem for Kn, which may be of independent interest. We also exhibit a sequence Gn of graphs with m(Gn)=n−b(n) such that Gn has O(nb(n)) edges and n vertices.