Factorisation is a hombrew for PSP to factor numbers and to say if it is a prime number.
Use
↑ / ↓: Increases or decreases the number of 1
← / →: Increases or decreases the number of 100
Left / Right: Multiplies or divides the number of 10
Χ: Allows you to enter a number with the virtual keyboard
□: Resets the number
select: Exit