 # Https://codeforces.com/problemset/problem/919

https://codeforces.com/problemset/problem/919/E
Can anyone help me in solving in this question? I am unable to understand the codeforces editorial. Brute force approach fails as the size of problem is 10^12(max)

/* I have only read the question, I’m not sure */
It is given a and p are coprime. That implies a^{p-1}\equiv1\ mod \ p.
Iterate through all i such that i<p-1. Now find modulo inverse of a^i\ mod\ p. This can be done in O(1), if you exponentiate a^{-1} directly. Now find ba^{-i}.Now n has to ba^{-i}\ mod\ p and i \ mod\ p-1 for this case. That implies n has to be a number such that n=px + ba^{-i}= (p-1)y + i for some integer x \ and\ y. One solution is x=y=i-ba^{-i}. Substitute this to find n\ mod \ p(p-1). Now count the number of occurences of this remainder in x.

actually the same problem is asked in the today’s gfg contest…

Yes. But they didn’t state that p is prime.

Input
The only line contains four integers a,b,p,x (2≤p≤10^6+3, 1≤a,b<p, 1≤x≤10^{12}). It is guaranteed that p is a prime.

I meant for the gfg problem.

yes , but after compairing gfg editorial with codeforces there is no change at all…

Could you please explain the editorial. J am unable to get it. @rishi2020 @epsilon_573

Learn chinese remainder theorem for solving this question I know Chinese remainder theorem. However i am not getting how that can be applied to this question. Any hints would be appreciated.

why looping till p is enough?

Looping should be for p(p-1).

I found this code but unable to understand the logic inside condition(a,b,p,x) function. Anyone explanation would be appreciated? Where does chinese remainder theorem used in this?

https://ide.geeksforgeeks.org/edoTXYe1J4

It’s the same as my solution.

def condition(a, b, p, x):
ans = 0
for i in range(1, p)://0 to p-1 is the same as 1 to p
now = b*inv(pow(a,i,p), p)%p//ba^-i
first = (p-1)*((i-now+p)%p)+i//x=y= i - ba^-1, n=px + ba^-i = (p-1)y + i
if first > x:
continue
ans += (x-first) // (p*(p-1))+1//count occureces of this mod p(p-1)
return ans

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Could you please explain this code? What concept is this? This is not Chinese remainder theorem. We are using Fermat’s little theorem and optimised (O(logn)) power function. All these things i know. I am badly stuck in that condition function. What is that algorithm? Please mention so that i can learn. If it is a modified form of Chinese remainder theorem or any other adhoc logic, please explain the code Why are we iterating till (p-1)?

We know that a^n \ mod \ p is periodic with period p-1. So we iterate over all possible n\ mod\ p-1. Let’s call that i.

Now we want na^n\ \equiv b\ mod\ p, where n\equiv i\ mod\ p-1.Therefore, it is equivalent to finding n such that, na^i\equiv b\ mod\ p.

That implies n\equiv ba^{-i}\ mod\ p.Since n is also i\ mod\ p-1 ,
n=px + ba^{-i}=(p-1)y+i.

One possible case is x=y=i-ba^{-i}.
We are using a bit of chinese remainder theorem to prove that if we know n\ mod\ p and n \ mod\ p-1, There is only one valid n\ mod\ p(p-1), since p and p-1 are always coprime.

So if we substitute x and y, We will get a valid n\ mod\ p(p-1).
Then we count the occurences of n\ mod\ p(p-1) in range x. We do this for all i and add up the answers.

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How can you say that n≡i mod p−1 ?

We iterate over all i from 1 to p-1., and n must be something mod p-1.

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I understood that a^n mod p is periodic with period p-1. But is this a property that
a^i mod p is same as a^n mod p where 1<=i<=p-1?