EigenCalc
One of the primary uses of eigenvalues and eigenvectors in structural geology is to calculate the principal axes of tensor quantities like stress, strain, or the orientation tensor. The actual numerical methods for calculating eigenvalues are beyond the background of most introductory structural geology students and, while spreadsheet programs like Microsoft Excel™ can calculate them via Visual Basic macros, or using the goal seek tool, doing so is not obvious and can be tedious (and can be subject to macro viruses).
I wrote EigenCalc to provide students with a simple tool to do these calculations. The use is very straightforward: enter a 3 x 3 symmetric matrix (the program makes it impossible to enter an asymmetric matrix) and press the Calculate button. EigenCalc then calculates the principal axes of the tensor (i.e., the eigenvalues) and the orientations of those axes (the eigenvectors) as direction cosines in the same coordinate system as the original matrix. Additionally, the program calculates the invariants of the matrix.
The user can save the complete analysis as a text file or can select the desired text from both text areas, and select copy. The results will be placed on the system clipboard, ready to be pasted into whatever program the user wants.
That's all there is to it!