Fading Coder

One Final Commit for the Last Sprint

Home > Tech > Content

Computing LQP Integer Partitions Using Generating Functions

Tech Apr 26 48

Consider a sequence of length (n) with associated value (F_n). This value can be interpreted as a count, particularly in tiling problems where each tile has a distinct color. This interpretation allows transformation into a solvable form.

The generating function for the tiling sequence is given by: [\frac{x}{1-x-x^2}]

The overall generating function becomes: [\frac{1}{1-\frac{x}{1-x-x^2}}]

Simplifying this expression by multiplying numerator and denominator by (1-x-x^2): [\frac{1-x-x^2}{1-2x-x^2}]

This can be rewritten as: [1+\frac{x}{1-2x-x^2}]

Performing partial fraction decomposition: [\frac{A}{x-x_1}+\frac{B}{x-x_2}]

Where: [\begin{cases}x_1=\sqrt{2}-1 \ x_2=-1-\sqrt{2} \ A=\frac{\sqrt{2}-1}{2\sqrt{2}} \ B=\frac{\sqrt{2}+1}{2\sqrt{2}} \end{cases}]

Rewriting in terms of reciporcal forms: [\frac{A/x_1}{x/x_1-1}+\frac{B/x_2}{x/x_2-1}]

Converting to negative forms suitable for series expansion: [\frac{-A/x_1}{1-x/x_1}+\frac{-B/x_2}{1-x/x_2}]

Extracting the coefficient of (x^n): [[x^n]\frac{-A/x_1}{1-x/x_1}+[x^n]\frac{-B/x_2}{1-x/x_2}]

This yields: [-\frac{A}{x_1^{n+1}}-\frac{B}{x_2^{n+1}}]

Substituting known values and factoring: [\frac{1}{2\sqrt{2}}\left((1+\sqrt{2})^n-(1-\sqrt{2})^n\right)]

For modular arithmetic with (P = 10^9 + 7), the square root of 2 modulo (P) is 940286407.

#include <bits/stdc++.h>
using namespace std;

template <typename T>
inline void read_input(T& value) {
    value = 0;
    char ch = getchar();
    int sign = 1;
    while(!('0' <= ch && ch <= '9')) {
        if(ch == '-') sign = -1;
        ch = getchar();
    }
    while('0' <= ch && ch <= '9') {
        value = value * 10 + ch - '0';
        ch = getchar();
    }
    value *= sign;
}

template <typename T, typename... Args>
inline void read_input(T& t, Args&... args) {
    read_input(t);
    read_input(args...);
}

const long long MOD = 1000000007;
const long long SQRT_2_MOD = 940286407;

long long modular_power(long long base, long long exp) {
    long long result = 1;
    while(exp > 0) {
        if(exp & 1) result = (result * base) % MOD;
        base = (base * base) % MOD;
        exp >>= 1;
    }
    return result;
}

int main() {
    long long n = 0;
    char digit;
    while(cin >> digit) {
        n = (n * 10 + digit - '0') % (MOD - 1);
    }
    
    long long term1 = modular_power(1 + SQRT_2_MOD, n);
    long long term2 = modular_power(1 - SQRT_2_MOD + MOD, n);
    long long difference = (term1 - term2 + MOD) % MOD;
    
    long long denominator_inverse = modular_power(2 * SQRT_2_MOD, MOD - 2);
    cout << (difference * denominator_inverse) % MOD << endl;
    
    return 0;
}

Related Articles

Understanding Strong and Weak References in Java

Strong References Strong reference are the most prevalent type of object referencing in Java. When an object has a strong reference pointing to it, the garbage collector will not reclaim its memory. F...

Implement Image Upload Functionality for Django Integrated TinyMCE Editor

Django’s Admin panel is highly user-friendly, and pairing it with TinyMCE, an effective rich text editor, simplifies content management significantly. Combining the two is particular useful for bloggi...

SBUS Signal Analysis and Communication Implementation Using STM32 with Fus Remote Controller

Overview In a recent project, I utilized the SBUS protocol with the Fus remote controller to control a vehicle's basic operations, including movement, lights, and mode switching. This article is aimed...

Leave a Comment

Anonymous

◎Feel free to join the discussion and share your thoughts.