The phishing problem can be specifically described as: given a set U and a subset family {S 1, S2, ..., Sn}, in which each Si is a non-empty subset of U, select an element from U, and if this element belongs to a Si, delete it together with Si. Repeat this process until u is an empty set. Is there a selection method so that at least one Si is selected at each step?
To solve the fishing problem, we usually need to use some concepts and methods in combinatorial mathematics, such as the basic operation of sets, the number of combinations, the principle of inclusion and exclusion, etc. By analyzing the characteristics and constraints of the problem, a mathematical model can be established and a solution can be derived.
Fishing problems are widely used in mathematical competitions, which can examine the logical thinking ability, mathematical reasoning ability and the application ability of combinatorial mathematics knowledge of contestants. At the same time, the phishing problem also has certain realistic background and application value, such as similar problems can be found in the fields of resource allocation and network optimization. Therefore, the problem of fishing is a kind of problem that attracts much attention in mathematics competition.