The mathematical fountain of youth! For those who slept through calculus, this is factorial notation having an existential crisis. When you calculate n! (factorial), you multiply all integers from 1 to n. So if you're 30, doing 30! gives you a number with more digits than atoms in the observable universe. Then taking F-1 (the inverse) brings you right back to 30.
It's basically math saying "congratulations, you've discovered your exact age through the most unnecessarily complicated method possible." Pure mathematician energy right there - spend 6 hours proving something we already knew.