1. A handheld computer device having a user interface to input a mathematical function comprising: a screen capable of displaying mathematical functions and a cursor; an input device capable of selecting positions of said cursor and moving said cursor horizontally or vertically on said screen; a processor for executing programming that provides a user interface comprising: a graph having horizontal and vertical lines, a constant function input for designating a horizontal constant function and an input point to move the constant function up and down on the graph, a linear mode input to select a linear function between two input points, at either side of the graph, and wherein the two input locations are movable up and down the graph based on user input.