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Hensel’s Lemma
  • Last Updated : 13 May, 2021

Hensel’s Lemma is a result that stipulates conditions for roots of polynomials modulo powers of primes to be “lifted” to roots modulo higher powers. The lifting method outlined in the proof is reminiscent of Newton’s Method for solving equations. Say the equations of the below type is to be solved:

(x^3 - a) mod \; p^k = 0

The idea is to use Hensel’s Lemma to solve this type of congruence. It is also known as Hensel’s “lifting” lemma, which is a result of modular arithmetic. If f is a polynomial function and p is a Prime Number, then if f(a1) = 0 (mod p) and [f'(a1)] mod p != 0, then it’s possible to “lift” this solution to the solution for f(x) = 0 (mod pk) by using the below recursion. Note that the [f'(a1)]-1 refers to the modular inverse of f ‘(a1) modulo p.

a_k + 1 = a_k - f(a_k) * [f'(a_1)]^{-1} (mod \; p^{k+1})

Example 1: 



x^3 - 3 = 0 (mod \; 25)

First, solve for (x = 2 is the solution for mod 5) 

=> x^3 - 3 \equiv 0 (mod \; 5)
=> X = 2 f'(2) \neq 0 (mod \; 5)
=> a_2 = 2 - f(2)[f'(2)]^{-1} mod \; 25
=> a_2 = 2 - 5 * 3 \; mod \; 25
=> a_2 = -13 \; mod \; 25
=> a_2 = 12 \; mod \; 25
 

Example 2: 

(x^3 - 3) \equiv (0 \; mod \; 49)

This has no solution as (x3 – 3) % mod 7 = 0 has no solution

Below is the implementation of Hensel’s Lemma

C++




// C++ program to illustrate the
// Hensel's Lemma
#include <bits/stdc++.h>
using namespace std;
 
// Consider f(x) = x ^ 3 - k,
// where k is a constant
 
// Function to find the modular
// inverse of a modulo m
int inv(int a, int m)
{
    int m0 = m, t, q;
    int x0 = 0, x1 = 1;
    if (m == 1)
        return 0;
 
    // Apply the Euclidean algorithm,
    // to find the modular inverse
    while (a > 1) {
        q = a / m;
        t = m;
        m = a % m;
        a = t;
        t = x0;
        x0 = x1 - q * x0;
        x1 = t;
    }
    if (x1 < 0)
        x1 += m0;
    return x1;
}
 
// Function to find the derivative of
// f(x) and f'(x) = 3 * (x ^ 2)
int derivative(int x)
{
    return 3 * x * x;
}
 
// Function to find the image of
// x in f(x) = x ^ 3 - k.
int Image(int x, int k)
{
    return x * x * x - k;
}
 
// Function to find the next power
// of the number
int next_power(int a_t, int t, int a1,
               int prime, int k)
{
    // Next power of prime for which
    // solution is to be found
    int power_p = (int)pow(prime, t + 1);
 
    // Using Hensel's recursion to
    // find the solution (next_a) for
    // next power of prime
    int next_a = (a_t
                  - Image(a_t, k)
                        * inv(derivative(a1),
                              prime))
                 % power_p;
 
    // If next_a < 0 return equivalent
    // positive remainder modulo p
    if (next_a < 0)
        return next_a += power_p;
 
    // Return the next power of a
    return next_a;
}
 
// Function to find the solution of
// the required exponent of prime
int powerOfPrime(int prime, int power,
                 int k, int a1)
{
    // The lemma does not work for
    // derivative of f(x) at a1
    if (derivative(a1) != 0) {
        int a_t = a1;
 
        // Looping from 1 to power
        // of prime whose solution
        // is to be found
        for (int p = 1; p < power; p++) {
            a_t = next_power(a_t, p,
                             a1, prime, k);
        }
 
        // Final answer after evaluating
        // all the exponents up till
        // the required exponent
        return a_t;
    }
 
    return -1;
}
 
// Driver Code
int main()
{
    int prime = 7, a1 = 3;
    int power = 2, k = 3;
 
    // Function Call
    cout << powerOfPrime(prime, power,
                         k, a1);
 
    return 0;
}

Java




// Java program to illustrate the
// Hensel's Lemma
import java.util.*;
 
class GFG{
 
// Consider f(x) = x ^ 3 - k,
// where k is a constant
 
// Function to find the modular
// inverse of a modulo m
static int inv(int a, int m)
{
    int m0 = m, t, q;
    int x0 = 0, x1 = 1;
     
    if (m == 1)
        return 0;
 
    // Apply the Euclidean algorithm,
    // to find the modular inverse
    while (a > 1)
    {
        q = a / m;
        t = m;
        m = a % m;
        a = t;
        t = x0;
        x0 = x1 - q * x0;
        x1 = t;
    }
    if (x1 < 0)
        x1 += m0;
         
    return x1;
}
 
// Function to find the derivative of
// f(x) and f'(x) = 3 * (x ^ 2)
static int derivative(int x)
{
    return 3 * x * x;
}
 
// Function to find the image of
// x in f(x) = x ^ 3 - k.
static int Image(int x, int k)
{
    return x * x * x - k;
}
 
// Function to find the next power
// of the number
static int next_power(int a_t, int t, int a1,
                      int prime, int k)
{
     
    // Next power of prime for which
    // solution is to be found
    int power_p = (int)Math.pow(prime, t + 1);
 
    // Using Hensel's recursion to
    // find the solution (next_a) for
    // next power of prime
    int next_a = (a_t - Image(a_t, k) *
                  inv(derivative(a1), prime)) %
                  power_p;
 
    // If next_a < 0 return equivalent
    // positive remainder modulo p
    if (next_a < 0)
        return next_a += power_p;
 
    // Return the next power of a
    return next_a;
}
 
// Function to find the solution of
// the required exponent of prime
static int powerOfPrime(int prime, int power,
                        int k, int a1)
{
     
    // The lemma does not work for
    // derivative of f(x) at a1
    if (derivative(a1) != 0)
    {
        int a_t = a1;
 
        // Looping from 1 to power
        // of prime whose solution
        // is to be found
        for(int p = 1; p < power; p++)
        {
            a_t = next_power(a_t, p,
                             a1, prime, k);
        }
 
        // Final answer after evaluating
        // all the exponents up till
        // the required exponent
        return a_t;
    }
    return -1;
}
 
// Driver Code
public static void main(String []args)
{
    int prime = 7, a1 = 3;
    int power = 2, k = 3;
     
    // Function Call
    System.out.print(powerOfPrime(prime, power,
                                  k, a1));
}
}
 
// This code is contributed by rutvik_56

C#




// C# program to illustrate the
// Hensel's Lemma
using System;
class GFG
{
 
// Consider f(x) = x ^ 3 - k,
// where k is a constant
 
// Function to find the modular
// inverse of a modulo m
static int inv(int a, int m)
{
    int m0 = m, t, q;
    int x0 = 0, x1 = 1;  
    if (m == 1)
        return 0;
 
    // Apply the Euclidean algorithm,
    // to find the modular inverse
    while (a > 1)
    {
        q = a / m;
        t = m;
        m = a % m;
        a = t;
        t = x0;
        x0 = x1 - q * x0;
        x1 = t;
    }
    if (x1 < 0)
        x1 += m0;      
    return x1;
}
 
// Function to find the derivative of
// f(x) and f'(x) = 3 * (x ^ 2)
static int derivative(int x)
{
    return 3 * x * x;
}
 
// Function to find the image of
// x in f(x) = x ^ 3 - k.
static int Image(int x, int k)
{
    return x * x * x - k;
}
 
// Function to find the next power
// of the number
static int next_power(int a_t, int t, int a1,
                      int prime, int k)
{
     
    // Next power of prime for which
    // solution is to be found
    int power_p = (int)Math.Pow(prime, t + 1);
 
    // Using Hensel's recursion to
    // find the solution (next_a) for
    // next power of prime
    int next_a = (a_t - Image(a_t, k) *
                  inv(derivative(a1), prime)) %
                  power_p;
 
    // If next_a < 0 return equivalent
    // positive remainder modulo p
    if (next_a < 0)
        return next_a += power_p;
 
    // Return the next power of a
    return next_a;
}
 
// Function to find the solution of
// the required exponent of prime
static int powerOfPrime(int prime, int power,
                        int k, int a1)
{
     
    // The lemma does not work for
    // derivative of f(x) at a1
    if (derivative(a1) != 0)
    {
        int a_t = a1;
 
        // Looping from 1 to power
        // of prime whose solution
        // is to be found
        for(int p = 1; p < power; p++)
        {
            a_t = next_power(a_t, p,
                             a1, prime, k);
        }
 
        // Final answer after evaluating
        // all the exponents up till
        // the required exponent
        return a_t;
    }
    return -1;
}
 
// Driver Code
public static void Main(string []args)
{
    int prime = 7, a1 = 3;
    int power = 2, k = 3;
     
    // Function Call
    Console.Write(powerOfPrime(prime, power,
                                  k, a1));
}
}
 
// This code is contributed by pratham76.

Javascript




<script>
// Javascript program to illustrate the
// Hensel's Lemma
 
// Consider f(x) = x ^ 3 - k,
// where k is a constant
  
// Function to find the modular
// inverse of a modulo m
function inv(a, m)
{
    let m0 = m, t, q;
    let x0 = 0, x1 = 1;
      
    if (m == 1)
        return 0;
  
    // Apply the Euclidean algorithm,
    // to find the modular inverse
    while (a > 1)
    {
        q = Math.floor(a / m);
        t = m;
        m = a % m;
        a = t;
        t = x0;
        x0 = x1 - q * x0;
        x1 = t;
    }
    if (x1 < 0)
        x1 += m0;
          
    return x1;
}
  
// Function to find the derivative of
// f(x) and f'(x) = 3 * (x ^ 2)
function derivative(x)
{
    return 3 * x * x;
}
  
// Function to find the image of
// x in f(x) = x ^ 3 - k.
function Image(x, k)
{
    return x * x * x - k;
}
  
// Function to find the next power
// of the number
function next_power(a_t, t, a1, prime, k)
{
      
    // Next power of prime for which
    // solution is to be found
    let power_p = Math.floor(Math.pow(prime, t + 1));
  
    // Using Hensel's recursion to
    // find the solution (next_a) for
    // next power of prime
    let next_a = (a_t - Image(a_t, k) *
                  inv(derivative(a1), prime)) %
                  power_p;
  
    // If next_a < 0 return equivalent
    // positive remainder modulo p
    if (next_a < 0)
        return next_a += power_p;
  
    // Return the next power of a
    return next_a;
}
  
// Function to find the solution of
// the required exponent of prime
function powerOfPrime(prime, power,
                        k, a1)
{
      
    // The lemma does not work for
    // derivative of f(x) at a1
    if (derivative(a1) != 0)
    {
        let a_t = a1;
  
        // Looping from 1 to power
        // of prime whose solution
        // is to be found
        for(let p = 1; p < power; p++)
        {
            a_t = next_power(a_t, p,
                             a1, prime, k);
        }
  
        // Final answer after evaluating
        // all the exponents up till
        // the required exponent
        return a_t;
    }
    return -1;
}
 
  // Driver Code
     
    let prime = 7, a1 = 3;
    let power = 2, k = 3;
      
    // Function Call
    document.write(powerOfPrime(prime, power,
                                  k, a1));
                                   
// This code is contributed by target_2.             
</script>
Output: 
6

 

Time Complexity: O(log N)
Auxiliary Space: O(1)

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