1. An information processing apparatus comprising: one or more hardware processors; a memory having stored thereon a lookup table indicating a correspondence between a variable and a function value of a single-variable function, wherein a product of a plurality of function values of the single-variable function approximates a function value of a multiple-variable function to reduce the size of the lookup table in the memory, and wherein the memory further has stored thereon instructions that, when executed by the one or more hardware processors, cause the information processing apparatus to: acquire a multidimensional input vector; derive a plurality of function values of the single-variable function with an element of the dimension as a variable for respective dimensions of the multidimensional input vector, by referring to the lookup table to increase the speed of execution of the instructions by the one or more hardware processors; calculate, for each dimension of the multidimensional input vector, a product of the derived function value and a predetermined coefficient corresponding to the dimension; calculate a total of products calculated for respective dimensions of the multidimensional input vector; classify the multidimensional input vector into a class by comparing the total of the products with a predetermined threshold; and output a classification index indicating the class of the multidimensional input vector.