In computability theory and computational complexity theory, a decision problem is a problem that can be posed as a

*yes*-*no**question*of the input values. Decision problems typically appear in mathematical*questions*of decidability, that is, the*question*of the existence of an effective method to determine the existence of some object or its membership in a set; some of the most important problems in mathematics are undecidable. For example, the problem "given two numbers x and y, does x evenly divide y?" is a decision problem. The answer can be either 'yes' or 'no', and depends upon the values of x and y.