Print numbers such that no two consecutive numbers are co-prime and every three consecutive numbers are co-prime
Last Updated :
06 Feb, 2023
Given an integer N, the task is to print N integers ? 109 such that no two consecutive of these integers are co-prime and every 3 consecutive are co-prime.
Examples:
Input: N = 3
Output: 6 15 10
Input: N = 4
Output: 6 15 35 14
Approach:
- We can just multiply consecutive primes and for the last number just multiply the gcd(last, last-1) * 2. We do this so that the (n – 1)th number, nth and 1st numbers can also follow the property mentioned in the problem statement.
- Another important part of the problem is the fact that the numbers should be ? 109. If you just multiply consecutive prime numbers, after around 3700 numbers, the value will cross 109. So we need to only use those prime numbers whose product won’t cross 109.
- To do this efficiently, consider a small number of primes, say the first 550 primes, and select them in a way such that on making a product no number gets repeated. We first choose every prime consecutively and then choose the primes with an interval of 2 and then 3 and so on. By doing that, we already make sure that no number gets repeated.
So we will select
5, 11, 17, …
Next time, we can start with 7 and select,
7, 13, 19, …
Below is the implementation of the above approach:
C++
#include <bits/stdc++.h>
using namespace std;
#define limit 1000000000
#define MAX_PRIME 2000000
#define MAX 1000000
#define I_MAX 50000
map< int , int > mp;
int b[MAX];
int p[MAX];
int j = 0;
bool prime[MAX_PRIME + 1];
void SieveOfEratosthenes( int n)
{
memset (prime, true , sizeof (prime));
for ( int p = 2; p * p <= n; p++) {
if (prime[p] == true ) {
for ( int i = p * p; i <= n; i += p)
prime[i] = false ;
}
}
for ( int p = 2; p <= n; p++) {
if (prime[p]) {
b[j++] = p;
}
}
}
int gcd( int a, int b)
{
if (b == 0)
return a;
return gcd(b, a % b);
}
void printSeries( int n)
{
SieveOfEratosthenes(MAX_PRIME);
int i, g, k, l, m, d;
int ar[I_MAX + 2];
for (i = 0; i < j; i++) {
if ((b[i] * b[i + 1]) > limit)
break ;
p[i] = b[i];
mp[b[i] * b[i + 1]] = 1;
}
d = 550;
bool flag = false ;
for (k = 2; (k < d - 1) && !flag; k++) {
for (m = 2; (m < d) && !flag; m++) {
for (l = m + k; l < d; l += k) {
if (((b[l] * b[l + k]) < limit)
&& (l + k) < d && p[i - 1] != b[l + k]
&& p[i - 1] != b[l]
&& mp[b[l] * b[l + k]] != 1) {
if (mp[p[i - 1] * b[l]] != 1) {
p[i] = b[l];
mp[p[i - 1] * b[l]] = 1;
i++;
}
}
if (i >= I_MAX) {
flag = true ;
break ;
}
}
}
}
for (i = 0; i < n; i++)
ar[i] = p[i] * p[i + 1];
for (i = 0; i < n - 1; i++)
cout << ar[i] << " " ;
g = gcd(ar[n - 1], ar[n - 2]);
cout << g * 2 << endl;
}
int main()
{
int n = 4;
printSeries(n);
return 0;
}
|
Java
import java.util.*;
class GFG
{
static int limit = 1000000000 ;
static int MAX_PRIME = 2000000 ;
static int MAX = 1000000 ;
static int I_MAX = 50000 ;
static HashMap<Integer,
Integer> mp = new HashMap<Integer,
Integer>();
static int []b = new int [MAX];
static int []p = new int [MAX];
static int j = 0 ;
static boolean []prime = new boolean [MAX_PRIME + 1 ];
static void SieveOfEratosthenes( int n)
{
for ( int i = 0 ; i < MAX_PRIME + 1 ; i++)
prime[i] = true ;
for ( int p = 2 ; p * p <= n; p++)
{
if (prime[p] == true )
{
for ( int i = p * p; i <= n; i += p)
prime[i] = false ;
}
}
for ( int p = 2 ; p <= n; p++)
{
if (prime[p])
{
b[j++] = p;
}
}
}
static int gcd( int a, int b)
{
if (b == 0 )
return a;
return gcd(b, a % b);
}
static void printSeries( int n)
{
SieveOfEratosthenes(MAX_PRIME);
int i, g, k, l, m, d;
int []ar = new int [I_MAX + 2 ];
for (i = 0 ; i < j; i++)
{
if ((b[i] * b[i + 1 ]) > limit)
break ;
p[i] = b[i];
mp.put(b[i] * b[i + 1 ], 1 );
}
d = 550 ;
boolean flag = false ;
for (k = 2 ; (k < d - 1 ) && !flag; k++)
{
for (m = 2 ; (m < d) && !flag; m++)
{
for (l = m + k; l < d; l += k)
{
if (((b[l] * b[l + k]) < limit) &&
mp.containsKey(b[l] * b[l + k]) &&
mp.containsKey(p[i - 1 ] * b[l]) &&
(l + k) < d && p[i - 1 ] != b[l + k] &&
p[i - 1 ] != b[l] &&
mp.get(b[l] * b[l + k]) != 1 )
{
if (mp.get(p[i - 1 ] * b[l]) != 1 )
{
p[i] = b[l];
mp.put(p[i - 1 ] * b[l], 1 );
i++;
}
}
if (i >= I_MAX)
{
flag = true ;
break ;
}
}
}
}
for (i = 0 ; i < n; i++)
ar[i] = p[i] * p[i + 1 ];
for (i = 0 ; i < n - 1 ; i++)
System.out.print(ar[i]+ " " );
g = gcd(ar[n - 1 ], ar[n - 2 ]);
System.out.print(g * 2 );
}
public static void main(String[] args)
{
int n = 4 ;
printSeries(n);
}
}
|
Python3
limit = 1000000000
MAX_PRIME = 2000000
MAX = 1000000
I_MAX = 50000
mp = {}
b = [ 0 ] * MAX
p = [ 0 ] * MAX
j = 0
prime = [ True ] * (MAX_PRIME + 1 )
def SieveOfEratosthenes(n):
global j
p = 2
while p * p < = n:
if (prime[p] = = True ):
for i in range (p * p, n + 1 , p):
prime[i] = False
p + = 1
for p in range ( 2 , n + 1 ):
if (prime[p]):
b[j] = p
j + = 1
def gcd(a, b):
if (b = = 0 ):
return a
return gcd(b, a % b)
def printSeries(n):
SieveOfEratosthenes(MAX_PRIME)
ar = [ 0 ] * (I_MAX + 2 )
for i in range (j):
if ((b[i] * b[i + 1 ]) > limit):
break
p[i] = b[i]
mp[b[i] * b[i + 1 ]] = 1
d = 550
flag = False
k = 2
while (k < d - 1 ) and not flag:
m = 2
while (m < d) and not flag:
for l in range (m + k, d, k):
if (((b[l] * b[l + k]) < limit) and
(l + k) < d and p[i - 1 ] ! = b[l + k] and
p[i - 1 ] ! = b[l] and
((b[l] * b[l + k] in mp) and
mp[b[l] * b[l + k]] ! = 1 )):
if (mp[p[i - 1 ] * b[l]] ! = 1 ):
p[i] = b[l]
mp[p[i - 1 ] * b[l]] = 1
i + = 1
if (i > = I_MAX):
flag = True
break
m + = 1
k + = 1
for i in range (n):
ar[i] = p[i] * p[i + 1 ]
for i in range (n - 1 ):
print (ar[i], end = " " )
g = gcd(ar[n - 1 ], ar[n - 2 ])
print (g * 2 )
if __name__ = = "__main__" :
n = 4
printSeries(n)
|
C#
using System;
using System.Collections.Generic;
class GFG
{
static int limit = 1000000000;
static int MAX_PRIME = 2000000;
static int MAX = 1000000;
static int I_MAX = 50000;
static Dictionary< int ,
int > mp = new Dictionary< int ,
int >();
static int []b = new int [MAX];
static int []p = new int [MAX];
static int j = 0;
static bool []prime = new bool [MAX_PRIME + 1];
static void SieveOfEratosthenes( int n)
{
for ( int i = 0; i < MAX_PRIME + 1; i++)
prime[i] = true ;
for ( int p = 2; p * p <= n; p++)
{
if (prime[p] == true )
{
for ( int i = p * p; i <= n; i += p)
prime[i] = false ;
}
}
for ( int p = 2; p <= n; p++)
{
if (prime[p])
{
b[j++] = p;
}
}
}
static int gcd( int a, int b)
{
if (b == 0)
return a;
return gcd(b, a % b);
}
static void printSeries( int n)
{
SieveOfEratosthenes(MAX_PRIME);
int i, g, k, l, m, d;
int []ar = new int [I_MAX + 2];
for (i = 0; i < j; i++)
{
if ((b[i] * b[i + 1]) > limit)
break ;
p[i] = b[i];
mp.Add(b[i] * b[i + 1], 1);
}
d = 550;
bool flag = false ;
for (k = 2; (k < d - 1) && !flag; k++)
{
for (m = 2; (m < d) && !flag; m++)
{
for (l = m + k; l < d; l += k)
{
if (((b[l] * b[l + k]) < limit) &&
mp.ContainsKey(b[l] * b[l + k]) &&
mp.ContainsKey(p[i - 1] * b[l]) &&
(l + k) < d && p[i - 1] != b[l + k] &&
p[i - 1] != b[l] &&
mp[b[l] * b[l + k]] != 1)
{
if (mp[p[i - 1] * b[l]] != 1)
{
p[i] = b[l];
mp.Add(p[i - 1] * b[l], 1);
i++;
}
}
if (i >= I_MAX)
{
flag = true ;
break ;
}
}
}
}
for (i = 0; i < n; i++)
ar[i] = p[i] * p[i + 1];
for (i = 0; i < n - 1; i++)
Console.Write(ar[i] + " " );
g = gcd(ar[n - 1], ar[n - 2]);
Console.Write(g * 2);
}
public static void Main(String[] args)
{
int n = 4;
printSeries(n);
}
}
|
Javascript
<script>
let limit = 1000000000
let MAX_PRIME = 2000000
let MAX = 1000000
let I_MAX = 50000
let mp = new Map();
let b = new Array(MAX);
let p = new Array(MAX);
let j = 0;
let prime = new Array(MAX_PRIME + 1);
function SieveOfEratosthenes(n)
{
prime.fill( true );
for (let p = 2; p * p <= n; p++) {
if (prime[p] == true ) {
for (let i = p * p; i <= n; i += p)
prime[i] = false ;
}
}
for (let p = 2; p <= n; p++) {
if (prime[p]) {
b[j++] = p;
}
}
}
function gcd(a, b)
{
if (b == 0)
return a;
return gcd(b, a % b);
}
function printSeries(n)
{
SieveOfEratosthenes(MAX_PRIME);
let i, g, k, l, m, d;
let ar = new Array(I_MAX + 2);
for (i = 0; i < j; i++) {
if ((b[i] * b[i + 1]) > limit)
break ;
p[i] = b[i];
mp[b[i] * b[i + 1]] = 1;
}
d = 550;
let flag = false ;
for (k = 2; (k < d - 1) && !flag; k++) {
for (m = 2; (m < d) && !flag; m++) {
for (l = m + k; l < d; l += k) {
if (((b[l] * b[l + k]) < limit)
&& (l + k) < d && p[i - 1] != b[l + k]
&& p[i - 1] != b[l] && mp[b[l] * b[l + k]] != 1) {
if (mp[p[i - 1] * b[l]] != 1) {
p[i] = b[l];
mp[p[i - 1] * b[l]] = 1;
i++;
}
}
if (i >= I_MAX) {
flag = true ;
break ;
}
}
}
}
for (i = 0; i < n; i++)
ar[i] = p[i] * p[i + 1];
for (i = 0; i < n - 1; i++)
document.write(ar[i] + " " );
g = gcd(ar[n - 1], ar[n - 2]);
document.write( g * 2 + "<br>" );
}
let n = 4;
printSeries(n);
</script>
|
Time Complexity: O(MAX_PRIME * I_MAX^2).
The time complexity of the above code is O(MAX_PRIME * I_MAX^2). Here MAX_PRIME is the maximum prime number that we have considered, I_MAX is the maximum number of prime numbers that can be produced and MAX is the maximum number that we have considered.
Space Complexity: O(MAX_PRIME + I_MAX).
The space complexity of the above code is O(MAX_PRIME + I_MAX). Here MAX_PRIME is the maximum prime number that we have considered and I_MAX is the maximum number of prime numbers that can be produced.
Another approach: List all the prime numbers up to 6 million by using the Sieve of Eratosthenes. We know the base condition i.e. N = 3 forms {6, 10, 15}.
So, we use these three values to generate further terms of the sequence.
Like {2, 3, 5}, these primes can not be used to generate sequences because they are already used in {6, 10, 15}. We also can’t use {7, 11}, which we’ll see later.
Now we have a prime list {13, 17, 19, 23, 29, ……}. We take the first prime and multiply it with 6, second with 15, third with 10, again 4th with 6, and so on…
13 * 6, 17 * 15, 19 * 10, 23 * 6, 29 * 15, ........upto N - 2 terms.
(N - 1)th term = (N - 1)th prime * 7.
Nth term = 7 * 11.
again, first term = first term * 11 to make 1st and last noncoprime.
For example, N = 5
6 * 11 * 13, 15 * 17, 10 * 19, 11 * 19, 7 * 11
Now we see that we can not use 7 and 11 from the list as these are used to generate the last and second last term.
Below is the implementation of the above approach:
C++
#include <bits/stdc++.h>
using namespace std;
const int MAX = 620000;
int prime[MAX];
void Sieve()
{
for ( int i = 2; i < MAX; i++) {
if (prime[i] == 0) {
for ( int j = 2 * i; j < MAX; j += i) {
prime[j] = 1;
}
}
}
}
void printSequence( int n)
{
Sieve();
vector< int > v, u;
for ( int i = 13; i < MAX; i++) {
if (prime[i] == 0) {
v.push_back(i);
}
}
if (n == 3) {
cout << 6 << " " << 10 << " " << 15;
return ;
}
int k;
for (k = 0; k < n - 2; k++) {
if (k % 3 == 0) {
u.push_back(v[k] * 6);
}
else if (k % 3 == 1) {
u.push_back(v[k] * 15);
}
else {
u.push_back(v[k] * 10);
}
}
k--;
u.push_back(v[k] * 7);
u.push_back(7 * 11);
u[0] = u[0] * 11;
for ( int i = 0; i < u.size(); i++) {
cout << u[i] << " " ;
}
}
int main()
{
int n = 4;
printSequence(n);
return 0;
}
|
Java
import java.util.*;
class GFG
{
static int MAX = 620000 ;
static int [] prime = new int [MAX];
static void Sieve()
{
for ( int i = 2 ; i < MAX; i++)
{
if (prime[i] == 0 )
{
for ( int j = 2 * i;
j < MAX; j += i)
{
prime[j] = 1 ;
}
}
}
}
static void printSequence( int n)
{
Sieve();
Vector<Integer> v = new Vector<Integer>();
Vector<Integer> u = new Vector<Integer>();
for ( int i = 13 ; i < MAX; i++)
{
if (prime[i] == 0 )
{
v.add(i);
}
}
if (n == 3 )
{
System.out.print( 6 + " " + 10 + " " + 15 );
return ;
}
int k;
for (k = 0 ; k < n - 2 ; k++)
{
if (k % 3 == 0 )
{
u.add(v.get(k) * 6 );
}
else if (k % 3 == 1 )
{
u.add(v.get(k) * 15 );
}
else
{
u.add(v.get(k) * 10 );
}
}
k--;
u.add(v.get(k) * 7 );
u.add( 7 * 11 );
u.set( 0 , u.get( 0 ) * 11 );
for ( int i = 0 ; i < u.size(); i++)
{
System.out.print(u.get(i) + " " );
}
}
public static void main(String[] args)
{
int n = 4 ;
printSequence(n);
}
}
|
Python3
MAX = 620000
prime = [ 0 ] * MAX
def Sieve():
for i in range ( 2 , MAX ):
if (prime[i] = = 0 ):
for j in range ( 2 * i, MAX , i):
prime[j] = 1
def printSequence (n):
Sieve()
v = []
u = []
for i in range ( 13 , MAX ):
if (prime[i] = = 0 ):
v.append(i)
if (n = = 3 ):
print ( 6 , 10 , 15 )
return
k = 0
for k in range (n - 2 ):
if (k % 3 = = 0 ):
u.append(v[k] * 6 )
elif (k % 3 = = 1 ):
u.append(v[k] * 15 )
else :
u.append(v[k] * 10 )
u.append(v[k] * 7 )
u.append( 7 * 11 )
u[ 0 ] = u[ 0 ] * 11
print ( * u)
if __name__ = = '__main__' :
n = 4
printSequence(n)
|
C#
using System;
using System.Collections.Generic;
class GFG
{
static int MAX = 620000;
static int [] prime = new int [MAX];
static void Sieve()
{
for ( int i = 2; i < MAX; i++)
{
if (prime[i] == 0)
{
for ( int j = 2 * i;
j < MAX; j += i)
{
prime[j] = 1;
}
}
}
}
static void printSequence( int n)
{
Sieve();
List< int > v = new List< int >();
List< int > u = new List< int >();
for ( int i = 13; i < MAX; i++)
{
if (prime[i] == 0)
{
v.Add(i);
}
}
if (n == 3)
{
Console.Write(6 + " " + 10 + " " + 15);
return ;
}
int k;
for (k = 0; k < n - 2; k++)
{
if (k % 3 == 0)
{
u.Add(v[k] * 6);
}
else if (k % 3 == 1)
{
u.Add(v[k] * 15);
}
else
{
u.Add(v[k] * 10);
}
}
k--;
u.Add(v[k] * 7);
u.Add(7 * 11);
u[0] = u[0] * 11;
for ( int i = 0; i < u.Count; i++)
{
Console.Write(u[i] + " " );
}
}
public static void Main(String[] args)
{
int n = 4;
printSequence(n);
}
}
|
Javascript
<script>
let MAX = 620000;
let prime = new Array(MAX);
for (let i=0;i<MAX;i++)
{
prime[i]=0;
}
function Sieve()
{
for (let i = 2; i < MAX; i++)
{
if (prime[i] == 0)
{
for (let j = 2 * i;
j < MAX; j += i)
{
prime[j] = 1;
}
}
}
}
function printSequence(n)
{
Sieve();
let v = [];
let u = [];
for (let i = 13; i < MAX; i++)
{
if (prime[i] == 0)
{
v.push(i);
}
}
if (n == 3)
{
document.write(6 + " " + 10 + " " + 15);
return ;
}
let k;
for (k = 0; k < n - 2; k++)
{
if (k % 3 == 0)
{
u.push(v[k] * 6);
}
else if (k % 3 == 1)
{
u.push(v[k] * 15);
}
else
{
u.push(v[k] * 10);
}
}
k--;
u.push(v[k] * 7);
u.push(7 * 11);
u[0] = u[0] * 11;
for (let i = 0; i < u.length; i++)
{
document.write(u[i] + " " );
}
}
let n = 4;
printSequence(n);
</script>
|
Time Complexity : O(n*log(n))
Space Complexity: O(n)
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