No.2181 LRM Question 2
Problem Statement
You are given positive integers $L, R, M$. Find the remainder when the following value is divided by $M$.
$\displaystyle \sum_{n=L}^{R} \left( \left( \prod_{i=1}^{n} \dfrac{ 1 }{ i^{2} } \right) \sum_{i=1}^{n-1} \left( \prod_{j=1}^{i} (n - j + 1)^{2} \prod_{j=i+1}^{n} j^{2} \right) \right)$
It is guaranteed that, under the constraints of this problem, the value of the above expression is always a non-negative integer.
Definition of $\sum, \prod$ (click to expand)
For a sequence $a = ( a_{1}, a_{2}, \dots, a_{n} )$ of length $n$, $\displaystyle \sum_{i=l}^{r} a_{i}$ and $\displaystyle \prod_{i=l}^{r} a_{i}$ are defined as follows.
$\displaystyle \sum_{i=l}^{r} a_{i} = a_{l} + a_{l+1} + \cdots + a_{r}$
$\displaystyle \prod_{i=l}^{r} a_{i} = a_{l} \times a_{l+1} \times \cdots \times a_{r}$
Here, $n,l,r$ are all integers satisfying $1 \leq l \leq r \leq n$.
Constraints
$2 \leq L \leq R \leq 10^{18}$
$R - L \leq 10^{5}$
$1 \leq M \leq 10^{6}$
All input values are integers.
Input
The input is given in the following format.
$L$ $R$ $M$
The $1$st line contains $L, R, M$ in this order, separated by single spaces.
Output
Output the answer in $1$ line.
Sample
Sample 1
Input
2 5 8
Output
4
Let $\displaystyle f(L,R) = \sum_{n=L}^{R} \left( \left( \prod_{i=1}^{n} \dfrac{ 1 }{ i^{2} } \right) \sum_{i=1}^{n-1} \left( \prod_{j=1}^{i} (n - j + 1)^{2} \prod_{j=i+1}^{n} j^{2} \right) \right)$, and let $$ \begin{aligned} &g_{1}(n) = \prod_{i=1}^{n} \dfrac{ 1 }{ i^{2} }, \\ &g_{2}(n) = \sum_{i=1}^{n-1} \left( \prod_{j=1}^{i} (n - j + 1)^{2} \prod_{j=i+1}^{n} j^{2} \right), \\ &h_{1}(i) = \prod_{j=1}^{i} (n - j + 1)^{2}, \\ &h_{2}(i) = \prod_{j=i+1}^{n} j^{2} \end{aligned} $$ Then, $$ g_{2}(n) = \sum_{i=1}^{n-1} ( h_{1}(i) \times h_{2}(i) ) $$ and $$ f(L,R) = \sum_{n=L}^{R} ( g_{1}(n) \times g_{2}(n) ) $$ hold.
In this case, $f(2,5) = 340$, and the remainder when this is divided by $8$ is $4$.
Sample 2
Input
2 2 2
Output
0
$f(2,2) = 4$, which is divisible by $2$.
Sample 3
Input
999999999999900000 1000000000000000000 1000000
Output
599998
The value of $f(L,R)$ can be very large.
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