## What is a Prime Number?

A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers. In simpler terms, a prime number is only divisible by 1 and itself. For example, 2, 3, 5, 7, and 11 are prime numbers, while 4, 6, 8, and 9 are not, as they can be divided evenly by numbers other than 1 and themselves.

**C Program**

```
#include <stdio.h>
// Function to check if a number is prime
int isPrime(int num) {
if (num <= 1) {
return 0; // 0 and 1 are not prime numbers
}
for (int i = 2; i * i <= num; i++) {
if (num % i == 0) {
return 0; // If the number is divisible by any i, it's not prime
}
}
return 1; // If no divisors found, the number is prime
}
int main() {
int number;
// Input the number
printf("Enter a number: ");
scanf("%d", &number);
// Check if the number is prime
if (isPrime(number)) {
printf("%d is a prime number.\n", number);
} else {
printf("%d is not a prime number.\n", number);
}
return 0;
}
```

This program defines a function `isPrime`

that takes an integer as an argument and returns 1 if the number is prime and 0 otherwise. The `main`

function takes user input, calls the `isPrime`

function, and prints the result.

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