| # Cardinality of a vector The cardinality of a vector is the number of non-zero elements in it. It is sometimes called the -norm of , although the cardinality function is not a norm. The cardinality is denoted , or . For example, the vector has cardinality . The cardinality function is useful in many problems, however it is difficult to optimize. In cardinality minimization problems, the -norm is often used as a surrogate.