# Largest number in [2, 3, .. n] which is co-prime with numbers in [2, 3, .. m]

• Last Updated : 07 Apr, 2021

Given two integers n and m, task is to find a number p which satisfy following conditions :
-> The number p should be less than or equal to n.
-> The number should be co-prime with all the integers from 2 to p(inclusive) i.e. the only positive integer which divides both of the numbers is 1.
Examples :

```Input :  n = 16, m = 3
Output : 13
Explanation : We need to find largest number
smaller than n and co-prime with all numbers
in set [2, 3, ... m] which is [2, 3] here.
Note that numbers {2, 4, 6, 8, ..} are not
co-prime with 2 and numbers {3, 6, 9, .. }
are not co-prime with 3.

Input : n = 6, m = 5
Output : -1 (Number doesn't exists)
Explanation : In this example 2 will cancel
out 2, 4, 6 and 3 will cancel out 3, 6
and 5 will cancel out 5. No number is left,
so the answer does not exists.```

Approach 1 : Create a list of numbers from 2 to n. And then run a loop of i = 2 to m and mark all numbers which are multiples of i. If i is already marked, don’t run loop as its multiples will be already marked. When the loop terminates run a loop from n to 2 until an unmarked number is found. The first unmarked number is the answer, if there is no unmarked number than number doesn’t exists. This approach takes O(n) auxiliary space so if the value of n is too large this method will not work.
Approach 2 : Run a loop from n to p+1 and check every number if it’s not divisible by any number between 2 and m.

## C++

 `#include ``using` `namespace` `std;` `// Returns true if i is co-prime with numbers``// in set [2, 3, ... m]``bool` `isValid(``long` `long` `int` `i, ``long` `long` `int` `m)``{``    ``// Running the loop till square root of n``    ``// to reduce the time complexity from n``    ``long` `long` `int` `sq_i = ``sqrt``(i);` `    ``// Find the minimum of square root of n``    ``// and m to run the loop until the smaller``    ``// one``    ``long` `long` `int` `sq = min(m, sq_i);` `    ``// Check from 2 to min(m, sqrt(n))``    ``for` `(``long` `long` `int` `j = 2; j <= sq; j++)``        ``if` `(i % j == 0)``            ``return` `false``;``    ` `    ``return` `true``;``}` `// Function to find the largest number less than n``// which is Co-prime with all numbers from 2 to m``void` `findLargestNum(``long` `long` `int` `n, ``long` `long` `int` `m)``{``    ``// Iterating from n to m+1 to find the number``    ``for` `(``long` `long` `int` `i = n; i > m; i--) {` `        ``// checking every number for the given``        ``// conditions``        ``if` `(isValid(i, m)) {``        ` `            ``// The first number which satisfy the``            ``// conditions is the answer``            ``cout << i << ``'\n'``;``            ``return``;``        ``}``    ``}` `    ``// If there is no number which satisfy the``    ``// conditions, then print number does not exist.``    ``cout << ``"Number Doesn't Exists\n"``;``}` `// Driver Program``int` `main()``{``    ``long` `long` `int` `n = 16, m = 3;``    ``findLargestNum(n, m);``    ``return` `0;``}`

## Java

 `// Java Largest number in [2, 3, .. n]``// which is co-prime with numbers``// in [2, 3, .. m]``import` `java.io.*;` `class` `GFG``{``    ``// Returns true if i is co-prime with numbers``    ``// in set [2, 3, ... m]``    ``static` `boolean` `isValid(``long` `i, ``long` `m)``    ``{``        ``// Running the loop till square root of n``        ``// to reduce the time complexity from n``        ``long` `sq_i = (``long``)Math.sqrt(i);``    ` `        ``// Find the minimum of square root of n``        ``// and m to run the loop until the smaller``        ``// one``        ``long` `sq = Math.min(m, sq_i);``    ` `        ``// Check from 2 to min(m, sqrt(n))``        ``for` `(``long` `j = ``2``; j <= sq; j++)``            ``if` `(i % j == ``0``)``                ``return` `false``;``        ` `        ``return` `true``;``    ``}``    ` `    ``// Function to find the largest number less than n``    ``// which is Co-prime with all numbers from 2 to m``    ``static` `void` `findLargestNum(``long` `n, ``long` `m)``    ``{``        ``// Iterating from n to m+1 to find the number``        ``for` `(``long` `i = n; i > m; i--) {``    ` `            ``// checking every number for the given``            ``// conditions``            ``if` `(isValid(i, m)) {``            ` `                ``// The first number which satisfy the``                ``// conditions is the answer``                ``System.out.println (i);``                ``return``;``            ``}``        ``}``    ` `        ``// If there is no number which satisfy the``        ``// conditions, then print number does not exist.``        ``System.out.println(``"Number Doesn't Exists"``);``    ``}``    ` `    ``// Driver Program``    ``public` `static` `void` `main (String[] args)``    ``{``        ``long` `n = ``16``, m = ``3``;``        ``findLargestNum(n, m);``            ` `    ``}``}``        ` `// This code is contributed by vt_m.`

## Python3

 `# Python3 code to find``# Largest number in``# [2, 3, .. n] which is``# co-prime with``# numbers in [2, 3, .. m]``import` `math` `# Returns true if i is``# co-prime with numbers``# in set [2, 3, ... m]``def` `isValid(i,m) :``    ` `    ``# Running the loop``    ``# till square root of n``    ``# to reduce the time``    ``# complexity from n``    ``sq_i ``=` `math.sqrt(i)` `    ``# Find the minimum of``    ``# square root of n``    ``# and m to run the loop``    ``# until the smaller``    ``# one``    ``sq ``=` `min``(m, sq_i)` `    ``# Check from 2 to``    ``# min(m, sqrt(n))``    ``for` `j ``in` `range``(``2``, sq ``+` `1``) :``        ``if` `(i ``%` `j ``=``=` `0``) :``            ``return` `False``    ` `    ``return` `True` `# def to find the``# largest number less``# than n which is Co-prime ``# with all numbers from``# 2 to m``def` `findLargestNum(n, m) :``    ` `    ``# Iterating from n to m+1``    ``# to find the number``    ``for` `i ``in` `range``(n, m, ``-``1``) :` `        ``# checking every number for``        ``# the given conditions``        ``if` `(isValid(i, m)) :``        ` `            ``# The first number``            ``# which satisfy the``            ``# conditions is the``            ``# answer``            ``print` `(``"{}\n"``.``format``(i));``            ``return` `    ``# If there is no number``    ``# which satisfy the``    ``# conditions, then print``    ``# number does not exist.``    ``print` `(``"Number Doesn't Exists\n"``)` `# Driver Code``n ``=` `16``m ``=` `3``findLargestNum(n, m)``    ` `# This code is contributed by``# Manish Shaw(manishshaw1)`

## C#

 `// C# Largest number in [2, 3, .. n]``// which is co-prime with numbers``// in [2, 3, .. m]``using` `System;` `class` `GFG``{``    ``// Returns true if i is co-prime``    ``// with numbers in set [2, 3, ... m]``    ``static` `bool` `isValid(``long` `i, ``long` `m)``    ``{``        ``// Running the loop till square root``        ``// of n to reduce the time complexity``        ``// from n``        ``long` `sq_i = (``long``)Math.Sqrt(i);``    ` `        ``// Find the minimum of square root``        ``// of n and m to run the loop until``        ``// the smaller one``        ``long` `sq = Math.Min(m, sq_i);``    ` `        ``// Check from 2 to min(m, sqrt(n))``        ``for` `(``long` `j = 2; j <= sq; j++)``            ``if` `(i % j == 0)``                ``return` `false``;``        ` `        ``return` `true``;``    ``}``    ` `    ``// Function to find the largest number``    ``// less than n which is Co-prime with``    ``// all numbers from 2 to m``    ``static` `void` `findLargestNum(``long` `n, ``long` `m)``    ``{``        ``// Iterating from n to m+1 to find the``        ``// number``        ``for` `(``long` `i = n; i > m; i--) {``    ` `            ``// checking every number for the given``            ``// conditions``            ``if` `(isValid(i, m)) {``            ` `                ``// The first number which satisfy the``                ``// conditions is the answer``                ``Console.WriteLine(i);``                ``return``;``            ``}``        ``}``    ` `        ``// If there is no number which satisfy``        ``// the conditions, then print number does``        ``// not exist.``        ``Console.WriteLine(``"Number Doesn't Exists"``);``    ``}``    ` `    ``// Driver Program``    ``public` `static` `void` `Main ()``    ``{``        ``long` `n = 55, m = 25;``        ``findLargestNum(n, m);``            ` `    ``}``}``        ` `// This code is contributed by vt_m.`

## PHP

 ` ``\$m``; ``\$i``--)``    ``{` `        ``// checking every number for``        ``// the given conditions``        ``if` `(isValid(``\$i``, ``\$m``))``        ``{``        ` `            ``// The first number``            ``// which satisfy the``            ``// conditions is the``            ``// answer``            ``echo` `\$i` `, ``"\n"``;``            ``return``;``        ``}``    ``}` `    ``// If there is no number``    ``// which satisfy the``    ``// conditions, then print``    ``// number does not exist.``    ``echo` `"Number Doesn't Exists\n"``;``}` `    ``// Driver Code``    ``\$n` `= 16; ``\$m` `= 3;``    ``findLargestNum(``\$n``, ``\$m``);``    ` `// This code is contributed by anuj_67``?>`

## Javascript

 ``

Output :

`13`

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