A number $n > 1$ is prime if there is no number $i$ with $1 < i < n$ such that $n$ is divisible by $i$.

• $2$ is prime because there are no numbers between $1$ and $2$;
• $3$ is prime because it is not divisible by $2$;
• $4$ is not prime because it is divisible by $2$;
• $5$ is prime because it is not divisible by either $2$, $3$, or $4$;
• $6$ is not prime because it is divisible by $2$ and $3$.

To check whether n is divisible by i, we can use remainder after division. In Python, this is obtained by the % operator.

n = 15
i = 4
print(n%i) # output: 3
i = 3
print(n%i) # output: 0

To check whether n is divisible by i, we can use n%i==0.