1. A computer implemented method for generating a character string to be displayed, the method comprising the steps of: searching a document on the basis of a keyword to retrieve a set (C) context strings (c); wherein the set (C) of context strings (c) includes the keyword; wherein the set (C) of context strings (c) comprises a first number (n1) of context strings (c); wherein n1 is an integer greater than or equal to 1; and obtaining one or more character strings (s) that maximize (argmax) a sum of areas, A(S,C), under the condition that a maximum number of character strings displayed is less than or equal to K; wherein K is an integer greater than or equal to 1; wherein each area is defined as a product of (1) a second number (n2) of context strings (c) in the set (C) of context strings (c) that have the character string (s) as a prefix, and (2) a length, len(s), of the character string (s).