How to use: Perform number theory operations including factorials and prime checking. Select Formula: Factorial (n! = n × (n-1) × ... × 1) or Prime Check (test if number is prime). Enter Number (positive integer). Results show: Factorial value (grows very fast: 5!=120, 10!=3,628,800) or Prime status (Yes/No). Prime numbers: divisible only by 1 and itself (2, 3, 5, 7, 11, 13, 17, 19, 23...). Factorials used in: permutations/combinations, probability calculations, series expansions, statistical formulas. Primes used in: cryptography (RSA encryption), number theory, computer science algorithms. Applications: lottery odds, password security, mathematical proofs.