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### pranay2063's blog

By pranay2063, 5 years ago, , Hi all,

While searching about inverse modulo, i got to know about a concise algorithm to find inverse modulo of numbers in range[1...n) under modulo m.

Time complexity of this approach is O(n).

Implementation of the algorithm:

r = 1; for (int i=2; i<n; ++i) r[i] = (m — (m/i) * r[m%i] % m) % m;  Comments (6)
 » This is the simplest I can think of:We want to prove that .    Now multiply both sides by . We assume that already.  And note that is the largest multiple of i that is not greater than m. In other words, .   Thus , and so . That is, r[i] is the modular inverse of i modulo m.There are some massive holes (mathematically) there, but should be easily patched up; but you want a simple explanation, not a completely correct one.
 » I was discussing with mgold earlier about another way to find the modular inverse of the first N numbers mod a prime number P.Find the factorial of the first N numbers, let's say fak[n] = n!, fak = 1 for k=1...N: fak[k] = fak[k-1] * k % P Find the modular inverse of N!. This is a result of Fermat Little Theorem and it can be computed in O(logP) ifak[N] = modpow(fak[N], P - 2) Now we can calculate the inverse of each factorial backwards in O(N). for k=N-1...0: ifak[k] = ifak[k+1] * (k+1) % P Most of the times I need to pre calculate the inverse of each of the first N numbers just to calculate the inverse of the factorial. So I can get C(n,k) in O(1). In that case we are done.But If you actually need the inverse of the first N numbers you can compute them: for k=1...N: inv[k] = ifak[k] * fak[k-1] % P Here is my code using this method: 35525761
•  » » » Can you explain how the ifak[k] = ifak[k+1]*(k+1)%P works? Notice that the inverse of factorials is computed in reverse order. You start by correctly computing the inverse factorial of $n$ which is $n!^{p - 2} mod(p)$: ifak[n] = modpow(fak[n], p - 2)Now that we now the inverse of $n!$ we can deduce smaller inverse in a similar fashion as we compute the factorials themselves $(k + 1)!^{-1} = (k! \cdot (k + 1))^{-1}$ $(k + 1)!^{-1} = k!^{-1} \cdot (k + 1)^{-1}$ $(k + 1)!^{-1} \cdot (k + 1) = k!^{-1}$So we deduce $k!^{-1}$ from $(k+1)!^{-1}$ easily using: ifak[k] = ifak[k + 1] * (k + 1) % p. More specifically, how modpow(fak[N],P-2) and ifak[k+1]*(k+1)%P produce the same result? Not sure what do you mean by this. modpow(fak[N],P-2) is only used to compute the inverse of $N!$. The recurrence ifak[k+1]*(k+1)%P is used to compute the remaining factorial inverses. $0 \le k \lt N$
•  » » » » What I meant earlier is that we could calculate inverse of each x! for x from 0 to N separately using modpow(fak[x],P-2) or we could do it faster with the iterative version, and I wanted to know how using both of them would give the same result.With the proof for the iterative step, it is very clear now, Thank you for this wonderful explanation!