# Open

This command is found on the File Menu and produces a file dialogue box for you to select the name of the file to load. This should be a simple text file of two columns of numbers representing (t,y(t)) values. The two columns should be separated by at least one space or a comma. More than two columns will be ignored. The values of t must be in increasing order with no repetition. A discrete function defined by you in this manner, as a sequence of values, is referred to in this help text as a User-Defined Function.

After loading, the graph of the function will be displayed. In this case, as the function expression is unknown, you cannot change the range specified for t or the sampling interval but may change the y-axis range.

If the function has a constant sampling interval then the Frequency Spectrum can be calculated. If, however, the values of t do not differ by a constant sampling interval then the Fourier Transform cannot be performed by the plotXpose algorithm and the Frequency spectrum will not be calculated.

If the sampling interval is constant then the Integral can be calculated using the Trapezoidal Rule or Simpson's Rule, however if the sampling interval is non-constant only the Trapezoidal rule is available.

The Derivative may be calculated using Forward-Difference formula or the Central-Difference formula but the Exact Derivative option is not available. Zero Find cannot be performed for user-defined functions but in the display mode you may tap on the t-axis, or input a value for t, near a point where the function appears to cross it, and an approximate value of the function, calculated using Linear Interpolation, will be displayed.