The statement:
Given three integers $$$n, k, p$$$, $$$(1 \leq k \leq n < p)$$$.
Count the number of array $$$a[]$$$ of size $$$k$$$ that satisfied
- $$$1 \leq a_1 < a_2 < \dots < a_k \leq n$$$
- $$$a_i \times a_j$$$ is perfect square $$$\forall 1 \leq i < j \leq n$$$
Since the number can be big, output it under modulo $$$p$$$.
For convenient, you can assume $$$p$$$ is a large constant prime $$$10^9 + 7$$$
Yet you can submit the problem for $$$k = 3$$$ here.
Notice that in this blog, we will solve for generalized harder variants
For original problem you can see in this blog [Tutorial] An interesting counting problem related to square product
Extra Tasks
These are harder variants, and generalization from the original problem. You can see more detail here
*Marked as solved only if tested with atleast $$$10^6$$$ queries
Solved A: Can we also use phi function or something similar to solve for $$$k = 2$$$ in $$$O(\sqrt{n})$$$ or faster ?
Solved B: Can we also use phi function or something similar to solve for general $$$k$$$ in $$$O(\sqrt{n})$$$ or faster ?
Solved C: Can we also solve the problem where there can be duplicate: $$$a_i \leq a_j\ (\forall\ i < j)$$$ and no longer $$$a_i < a_j (\forall\ i < j)$$$ ?
Solved D: Can we solve the problem where there is no restriction between $$$k, n, p$$$ ?
Solved E: Can we solve for negative integers, whereas $$$-n \leq a_1 < a_2 < \dots < a_k \leq n$$$ ?
Solved F: Can we solve for a specific range, whereas $$$L \leq a_1 < a_2 < \dots < a_k \leq R$$$ ?
G: Can we solve for cube product $$$a_i \times a_j \times a_k$$$ effectively ?
H: Can we solve if it is given $$$n$$$ and queries for $$$k$$$ ?
I: Can we solve if it is given $$$k$$$ and queries for $$$n$$$ ?
J: Can we also solve the problem where there are no order: Just simply $$$1 \leq a_i \leq n$$$ ?
K: Can we also solve the problem where there are no order: Just simply $$$0 \leq a_i \leq n$$$ ?
M: Can we solve for $$$q$$$-product $$$a_{i_1} \times a_{i_2} \times \dots \times a_{i_q} = x^q$$$ (for given constant $$$q$$$) ?
N: Given $$$0 \leq \delta \leq n$$$, can we also solve the problem when $$$1 \leq a_1 \leq a_1 + \delta + \leq a_2 \leq a_2 + \delta \leq \dots \leq a_k \leq n$$$ ?
O: What if the condition is just two nearby elements and not all pairs. Or you can say $$$a_i \times a_{i+1} \forall 1 \leq i < n$$$ is a perfect square ?
A better solution for k = 2
Extra task A
Idea
In the above approach, we fix $$$u$$$ as a squarefree and count $$$p^2$$$.
But what if I fix $$$p^2$$$ to count $$$u$$$ instead ?
Yet you can see that the first loop now is $$$O(\sqrt{n})$$$, but it will still $$$O(n)$$$ total because of the second loop
Approach
Let $$$f(n)$$$ is the number of pair $$$(a, b)$$$ that $$$1 \leq a < b \leq n$$$ and $$$(a, b, n)$$$ is a three-term geometric progression.
Let $$$g(n)$$$ is the number of pair $$$(a, b)$$$ that $$$1 \leq a \leq b \leq n$$$ and $$$(a, b, n)$$$ is a three-term geometric progression.
Let $$$F(n) = \overset{n}{\underset{p=1}{\Large \Sigma}} f(p)$$$.
So it is no hard to prove that $$$g(n) = f(n) + 1$$$.
This interesting sequence $$$g(n)$$$ is A000188, having many properties, such as
- Number of solutions to $$$x^2 \equiv 0 \pmod n$$$.
- Square root of largest square dividing $$$n$$$.
- Max $$$gcd \left(d, \frac{n}{d}\right)$$$ for all divisor $$$d$$$.
Well, to make the problem whole easier, I gonna skip all the proofs to use this property (still, you can use the link in the sequence for references).
$$$g(n) = \underset{d^2 | n}{\Large \Sigma} \phi(d)$$$.
From this property, we can solve the problem in $$$O(\sqrt{n})$$$.
Yet this paper also takes you to something similar.
Implementation
A better solution for general k
Extra task B
Algorithm
Let $$$f_k(n)$$$ is the number of set $$$(a_1, a_2, \dots, a_k, n)$$$ that $$$1 \leq a_1 < a_2 < \dots < a_k \leq n$$$ and $$$(a_1, a_2, \dots, a_k, n)$$$ is a $$$(k+1)$$$-term geometric progression.
Let $$$g_k(n)$$$ is the number of set $$$(a_1, a_2, \dots, a_k, n)$$$ that $$$1 \leq a_1 \leq a_2 \leq \dots \leq a_k \leq n$$$ and $$$(a_1, a_2, \dots, a_k, n)$$$ is a $$$(k+1)$$$-term geometric progression.
Let $$$F_k(n) = \overset{n}{\underset{p=1}{\Large \Sigma}} f_k(p)$$$.
Let $$$s_k(n)$$$ is the number of way to choose $$$p^2$$$ among those $$$k$$$ numbers when you fix squarefree $$$u$$$ (though we are doing in reverse).
Implementation
Complexity
The complexity of the first implementation is $$$O(\sqrt{n} \log \sqrt{n})$$$
The complexity of the second implementation is $$$O(\sqrt{n} \log \log \sqrt{n})$$$
Solution for duplicates elements in array
Extra task C
Idea
It is no hard to proove that we can use the same algorithm as described in task A, B or in original task.
Using the same algorithm, the core of calculating is to find out the number of non-decreasing integer sequence of size $$$k$$$ where numbers are in $$$[1, n]$$$.
Can you proove it ?
Now it is done, just that it
The idea is the same as what clyring described here but represented in the other way
Implementation
Complexity
In the first implementation it is obviously linear.
The second and third implementation is also easy to show its complexity
Sadly, since $$$k \leq n$$$. We also conclude that the complexity is $$$O(n)$$$, and even worse it also contains large constant factor compared to that in the first implementation.
But it is still effecient enough to solve problem where $$$k$$$ is small.
Solution when there are no restriction between k, n, p
Extra task D
Idea
So first of all, the result do depend on how you calculate binomial coefficient but they are calculated independently even if you can somehow manage to use the for loop of binomial coeffient go first.
Therefore even if there is no restriction between $$$k, n, p$$$, the counting part and the algorithm doesnt change.
You just need to change how you calculate binomial coefficient, and that is all for this task.
Let just ignore the fact that though this need more detail, but as the blog is not about nck problem I will just make it quick
For large prime $$$p > max(n, k)$$$
- Just using normal combinatorics related to factorial (since $$$p > max(n, k)$$$ nothing will affect the result)
- For taking divides under modulo you can just take modular inversion (as a prime always exist such number)
- Yet this is standard problem, just becareful of the overflow part
- You can also optimize by precalculating factorial, inversion number and inversion factorial in linear too
For general prime $$$p$$$
- We can just ignore factors $$$p$$$ in calculating $$$n!$$$.
- You also need to know how many times factor $$$p$$$ appears in $$$1 \dots n$$$
- Then combining it back when calculating for the answer.
- If we dont do this $$$n!$$$ become might divides some factors of $$$p$$$.
- By precalculation you can answer queries in $$$O(1)$$$
For squarefree $$$p$$$
- Factorize $$$p = p_1 \times p_2 \times p_q$$$ that all $$$p_i$$$ is prime.
- Ignore all factors $$$p_i$$$ when calculate $$$n!$$$.
- Remember to calculate how many times factors $$$p_i$$$ appear in $$$1 \dots n$$$.
- When query for the answer we just combine all those part back.
- Remember you can just take modulo upto $$$\phi(p)$$$ which you can also calculate while factorizing $$$p$$$.
- Remember that $$$n!$$$ must not divides any factor $$$p_i$$$ otherwise you will get wrong answer.
- By precalculation you can answer queries in $$$O(\log p)$$$
For general positive modulo $$$p$$$
- Factorize $$$p = p_1^{f_1} \times p_2^{f_2} \times p_q^{f_q}$$$ that all $$$p_i$$$ is unique prime.
- We calculate $$$C(n, k)$$$ modulo $$$p_i^{f_i}$$$ for each $$$i = 1 \dots q$$$.
- To do that, we need to calculate $$$n!$$$ modulo $$$p_i^{f_i}$$$ which is described here.
- To get the final answer we can use CRT.
- Yet this is kinda hard to code and debug also easy to make mistake so you must becareful
- I will let the implementation for you lovely readers.
- Yet depends on how you calculate stuffs that might increase your query complexity
- There are few (effective or atleast fully correct) papers about this but you can read the one written here
Implementation
Complexity
In the first implementation it is obviously linear.
And for the second implementation.
So you got $$$O(n \times \log p + \sqrt{p})$$$ in final.
Though you can still optimize this but by doing that why dont you just go straight up to solve for non squarefree $$$p$$$ too ?
Solution when numbers are also bounded by negative number
Extra task E
Idea
Yet this is the same as extra task C where only the counting part should be changed.
As we only care about integer therefore let not use complex math into this problem.
If there exist a negative number and a positive number, the product will be negative thus the sequence will not satisfied.
Becareful, there are the zeros too.
When the numbers are all unique, or $$$-n \leq a_1 < a_2 < \dots < a_k \leq n$$$
Thus give us the formula of $$$task_E(n, k) = 2 \times task_B(n, k) + 2 \times task_B(n, k - 1)$$$.
Remember that when $$$k = 0$$$ the answer is $$$0$$$ otherwise you might somewhat having wrong result for negative number in binomial coefficients formula
So what if I mix the problem with task C too ?
When the numbers can have duplicates, or $$$-n \leq a_1 \leq a_2 \leq \dots \leq a_k \leq n$$$
Yet once again you can simplified it with less cases for easier calculation.
Thus give us the formula of $$$task_E(n, k) = 1 + 2 \times \overset{k}{\underset{t = 1}{\Large \Sigma}} task_B(n, t)$$$.
But this give you a $$$O(k)$$$ solution.
You can do better with math
Implementation
And for duplicates (mixed with task C), we have:
Complexity
So.. you might be tired of calculating complexity again and again for those are too familar to you.
So I gonna skip this as the proof is as the same as what you can read above.
But as a bonus, you optimize the problem using this
Then you can solve the problem in $$$O(\sqrt{n} \log \log \sqrt{n} + \sqrt{p} \log^q \ p)$$$ ($$$q$$$ is depended on how you implement it).
Conclusion
For unique array, the complexity is $$$O(\sqrt{n} \log \log \sqrt{n})$$$
For duplicate array, the complexity is $$$O(\sqrt{n} \log \log \sqrt{n} + k)$$$
For duplicate array and small prime $$$p > max(n, k)$$$, the complexity is $$$O(\sqrt{n} \log \log \sqrt{n} + \sqrt{p} \log^q \ p)$$$
Solution when numbers are also bounded by a specific range
Extra task F
Algorithm
We split into 4 cases
You can easilier solve for each cases in linear
Now assumming $$$1 \leq L \leq R$$$, here is how we solve it.
As a simple approach, we can just do like original problem for general $$$k$$$.
We just need to iterative for each fixed squarefree $$$u$$$ and count the number of way to select $$$p^2$$$ as usual but a bit of change.
By doing so trivially we can have the complexity of $$$O((R - L + 1) \log R)$$$ time and $$$O(R)$$$ space.
Is it over ? Cant we do better ?
So what is that the better solution and how do we implement it steps by steps ?
Implementation
Let $$$Z = max(|L|, |R|)$$$
Complexity
Well, you might feel bored of long chain of proving for just a simple complexity, now I just make it quick.
Which part you find hard to understand, you can comment below if you cant accept this as an exercise.
For the first implemenetation it is obviously linear time in $$$max(|L|, |R|)$$$ and linear space in $$$|L - R|$$$
For the second implementation it is harder to prove. Yet it is bounded by the complexity of factorize each number in range $$$[L, R]$$$.
For the third implementation it is based on the fact that:
- The sum of inversion of prime squared is constant.
- Each number $$$x$$$ will not be factorize more than $$$O(\log(\sqrt{x}))$$$.
- You can precalculate binomial coefficient in linear and query for constant time.
- You can sieve in linear, and only sieve to the square root of $$$R$$$.
- For each number in the range we care, we just need to visit once.
Yet you can furthur optimize it to $$$O\left(\frac{\sqrt{R} \log \log \sqrt{R}}{\log \sqrt{R}} + \frac{R - L}{\pi^2}\right)$$$.
*I am working on a $$$O {\Large ( } Õ(\sqrt{R}) + Õ(\sqrt{R - L}) {\Large ) }$$$ solution right now as it is much harder to apply the same difficult math idea described in task B.
Contribution
Yurushia for pointing out the linear complexity of squarefree sieve.
clyring for fixing typos, and the approach for tasks A, B, C, D, E, G, H, J.
errorgorn for adding details, and the approach for task F, J, M, O, better complexity for C, E.
Lihwy, jalsol for combinatorics calculation and the proof of stars and bars in task C.
Editorial Slayers Team Lyde DeMen100ns Duy_e OnionEgg QuangBuiCPP _FireGhost_ for reviewing, fixing typos and feed backs.