Hi everyone!
I'm really facing problems in when to use modular inverse. I know that you should use it when calculating the following
(a / b)%p = (a * bp - 2)%p [Only when p is prime]
I know there is a case when subtracting, but I don't know how you do it exactly, can you tell me how? And are there any other cases?
Thanks in advance.
Remember, the modular inverse of some number a modulo m, is a number b such that:
a·b ≡ 1 (mod m)
That's why you use it when you have to do divisions modulo some number .
However, a number a has inverse modulo m iff they are coprime (gcd(a, m) = 1)
If m is prime, you can apply the formula you mentioned in your post. Otherwise, you have to find the modular inverse using another method (for example, Extended GCD)
I don't know what do you mean with "case when subtracting". If you have to perform a subtraction between two numbers modulo m, there's no need to use the modular inverse.