The TI-68 is a somewhat odd calculator. Texas Instruments produced many "almost programmables"; calculators with a facility to store keystrokes, but a very small program memory, and no program control instructions such as conditional or unconditional branching. The TI-68 qualifies as an "almost programmable" except that its prgram memory is fairly large; a total of 55 storage registers can be converted into approximately 440 program steps, in the form of up to 26 programs. Yet, no branches, no loops, no conditionals, not even a facility to call a second program from within a first (given that each individual program is limited to 79 program steps, this is a severe restriction.)

It is also a very fragile beast, difficult to repair. You almost need a stick of dynamite in order to open the plastic case (I have not been able to open it without causing some structural damage.) Inside, the LCD display is connected to the main circuit board using a piece of flexible circuit that is attached by some adhesive. If this circuit starts to peel off for any reason, the damage is almost impossible to repair.

Because of the limitation of 79 program steps per formula, it is not possible to program the Gamma function into this calculator, at least not without the use of variables for constants (the calculator in this case will inelegantly prompt for the value of each variable used, every time you run the program) or multiple programs (which would need to be called explicitly by the user.) Instead, here is a solution for Stirling's formula:

S=(X+1) yxX×√(2×π×(X+1))÷ e^(X+1)×(1+1÷12÷(X+1))