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Giuga numbers

From Rosetta Code
Giuga numbers is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
Definition

A Giuga number is a composite number n which is such that each of its distinct prime factors f divide (n/f - 1) exactly.

All known Giuga numbers are even though it is not known for certain that there are no odd examples.

Example

30 is a Giuga number because its distinct prime factors are 2, 3 and 5 and:

  • 30/2 - 1 = 14 is divisible by 2
  • 30/3 - 1 = 9 is divisible by 3
  • 30/5 - 1 = 5 is divisible by 5


Task

Determine and show here the first four Giuga numbers.

Stretch

Determine the fifth Giuga number and any more you have the patience for.

References




AWK[edit]

 
# syntax: GAWK -f GIUGA_NUMBER.AWK
BEGIN {
n = 3
stop = 4
printf("Giuga numbers 1-%d:",stop)
while (count < stop) {
if (is_giuga(n)) {
count++
printf(" %d",n)
}
n++
}
printf("\n")
exit(0)
}
function is_giuga(m, f,l,n) {
n = m
f = 2
l = sqrt(n)
while (1) {
if (n % f == 0) {
if (((m / f) - 1) % f != 0) { return(0) }
n /= f
if (f > n) { return(1) }
}
else {
if (++f > l) { return(0) }
}
}
}
 
Output:
Giuga numbers 1-4: 30 858 1722 66198

C++[edit]

Brute force[edit]

Based on the Go solution. Takes 26 minutes on my system (Intel Core i5 3.2GHz).

#include <iostream>
 
// Assumes n is even with exactly one factor of 2.
bool is_giuga(unsigned int n) {
unsigned int m = n / 2;
auto test_factor = [&m, n](unsigned int p) -> bool {
if (m % p != 0)
return true;
m /= p;
return m % p != 0 && (n / p - 1) % p == 0;
};
if (!test_factor(3) || !test_factor(5))
return false;
static constexpr unsigned int wheel[] = {4, 2, 4, 2, 4, 6, 2, 6};
for (unsigned int p = 7, i = 0; p * p <= m; ++i) {
if (!test_factor(p))
return false;
p += wheel[i & 7];
}
return m == 1 || (n / m - 1) % m == 0;
}
 
int main() {
std::cout << "First 5 Giuga numbers:\n";
// n can't be 2 or divisible by 4
for (unsigned int i = 0, n = 6; i < 5; n += 4) {
if (is_giuga(n)) {
std::cout << n << '\n';
++i;
}
}
}
Output:
First 5 Giuga numbers:
30
858
1722
66198
2214408306

Faster version[edit]

Library: Boost
Translation of: Wren
#include <boost/rational.hpp>
 
#include <algorithm>
#include <cstdint>
#include <iostream>
#include <vector>
 
using rational = boost::rational<uint64_t>;
 
bool is_prime(uint64_t n) {
if (n < 2)
return false;
if (n % 2 == 0)
return n == 2;
if (n % 3 == 0)
return n == 3;
for (uint64_t p = 5; p * p <= n; p += 4) {
if (n % p == 0)
return false;
p += 2;
if (n % p == 0)
return false;
}
return true;
}
 
uint64_t next_prime(uint64_t n) {
while (!is_prime(n))
++n;
return n;
}
 
std::vector<uint64_t> divisors(uint64_t n) {
std::vector<uint64_t> div{1};
for (uint64_t i = 2; i * i <= n; ++i) {
if (n % i == 0) {
div.push_back(i);
if (i * i != n)
div.push_back(n / i);
}
}
div.push_back(n);
sort(div.begin(), div.end());
return div;
}
 
void giuga_numbers(uint64_t n) {
std::cout << "n = " << n << ":";
std::vector<uint64_t> p(n, 0);
std::vector<rational> s(n, 0);
p[2] = 2;
p[1] = 2;
s[1] = rational(1, 2);
for (uint64_t t = 2; t > 1;) {
p[t] = next_prime(p[t] + 1);
s[t] = s[t - 1] + rational(1, p[t]);
if (s[t] == 1 || s[t] + rational(n - t, p[t]) <= rational(1)) {
--t;
} else if (t < n - 2) {
++t;
uint64_t c = s[t - 1].numerator();
uint64_t d = s[t - 1].denominator();
p[t] = std::max(p[t - 1], c / (d - c));
} else {
uint64_t c = s[n - 2].numerator();
uint64_t d = s[n - 2].denominator();
uint64_t k = d * d + c - d;
auto div = divisors(k);
uint64_t count = (div.size() + 1) / 2;
for (uint64_t i = 0; i < count; ++i) {
uint64_t h = div[i];
if ((h + d) % (d - c) == 0 && (k / h + d) % (d - c) == 0) {
uint64_t r1 = (h + d) / (d - c);
uint64_t r2 = (k / h + d) / (d - c);
if (r1 > p[n - 2] && r2 > p[n - 2] && r1 != r2 &&
is_prime(r1) && is_prime(r2)) {
std::cout << ' ' << d * r1 * r2;
}
}
}
}
}
std::cout << '\n';
}
 
int main() {
for (uint64_t n = 3; n < 7; ++n)
giuga_numbers(n);
}
Output:
n = 3: 30
n = 4: 1722 858
n = 5: 66198
n = 6: 24423128562 2214408306

FreeBASIC[edit]

Function isGiuga(m As Uinteger) As Boolean
Dim As Uinteger n = m
Dim As Uinteger f = 2, l = Sqr(n)
Do
If n Mod f = 0 Then
If ((m / f) - 1) Mod f <> 0 Then Return False
n /= f
If f > n Then Return True
Else
f += 1
If f > l Then Return False
End If
Loop
End Function
 
Dim As Uinteger n = 3, c = 0, limit = 4
Print "The first "; limit; " Giuga numbers are: ";
Do
If isGiuga(n) Then c += 1: Print n; " ";
n += 1
Loop Until c = limit
Output:
The first 4 Giuga numbers are: 30  858  1722  66198

Go[edit]

Translation of: Wren

I thought I'd see how long it would take to 'brute force' the fifth Giuga number and the answer (without using parallelization, Core i7) is about 1 hour 38 minutes.

package main
 
import "fmt"
 
var factors []int
var inc = []int{4, 2, 4, 2, 4, 6, 2, 6}
 
// Assumes n is even with exactly one factor of 2.
// Empties 'factors' if any other prime factor is repeated.
func primeFactors(n int) {
factors = factors[:0]
factors = append(factors, 2)
last := 2
n /= 2
for n%3 == 0 {
if last == 3 {
factors = factors[:0]
return
}
last = 3
factors = append(factors, 3)
n /= 3
}
for n%5 == 0 {
if last == 5 {
factors = factors[:0]
return
}
last = 5
factors = append(factors, 5)
n /= 5
}
for k, i := 7, 0; k*k <= n; {
if n%k == 0 {
if last == k {
factors = factors[:0]
return
}
last = k
factors = append(factors, k)
n /= k
} else {
k += inc[i]
i = (i + 1) % 8
}
}
if n > 1 {
factors = append(factors, n)
}
}
 
func main() {
const limit = 5
var giuga []int
// n can't be 2 or divisible by 4
for n := 6; len(giuga) < limit; n += 4 {
primeFactors(n)
// can't be prime or semi-prime
if len(factors) > 2 {
isGiuga := true
for _, f := range factors {
if (n/f-1)%f != 0 {
isGiuga = false
break
}
}
if isGiuga {
giuga = append(giuga, n)
}
}
}
fmt.Println("The first", limit, "Giuga numbers are:")
fmt.Println(giuga)
}
Output:
The first 5 Giuga numbers are:
[30 858 1722 66198 2214408306]

J[edit]

We can brute force this task building a test for giuga numbers and checking the first hundred thousand integers (which takes a small fraction of a second):

giguaP=: {{ (1<y)*(-.1 p:y)**/(=<.) y ((_1+%)%]) q: y }}"0
   1+I.giguaP 1+i.1e5
30 858 1722 66198

These numbers have some interesting properties but there's an issue with guaranteeing correctness of more sophisticated approaches. That said, here's a translation of the pari/gp implementation on the talk page:

divisors=: [: /:[email protected], */&>@{@((^ [email protected]>:)&.>/)@q:~&__
 
giuga=: {{
r=. i.0
p=. (2) 0 1} s=. 1r2,}.(2>.y-1+t=.1)$0
while. t do.
p=. p t}~ 4 p:t{p
s=. s t}~ (s{~t-1)+1%t{p
if. (1=t{s) +. 1 >: (t{s)+(y-t+1)%t{p do.
t=. t-1
elseif. t<y-3 do.
p=. p (t+1)}~ (p{~t) >. (%-.)s{~t
t=. t+1
else.
'c d'=. 2 x: s{~y-3
dc=. d-c
k=. (d^2)-dc
for_h. ({.~ <[email protected]:@>:@#) f=. divisors k do.
if. 0=dc|h+d do.
if. 0=dc|dkh=. d+k%h do.
py3=. p{~y-3
if. py3 < r1=. (h+d)%dc do.
if. py3 < r2=. dkh%dc do.
if. r1~:r2 do.
if. 1 p: r1 do.
if. 1 p: r2 do.
r=. r, d*r1*r2
end.
end.
end.
end.
end.
end.
end.
end.
end.
end.
r
}}
Example use:
   giuga 1
 
giuga 2
 
giuga 3
30
giuga 4
1722 858
giuga 5
66198
giuga 6
24423128562 2214408306

Julia[edit]

using Primes
 
isGiuga(n) = all(f -> f != n && rem(n ÷ f - 1, f) == 0, factor(Vector, n))
 
function getGiuga(N)
gcount = 0
for i in 4:typemax(Int)
if isGiuga(i)
println(i)
(gcount += 1) >= N && break
end
end
end
 
getGiuga(4)
 
Output:
30      
858 
1722
66198

Ad hoc faster version[edit]

using Primes
 
function getgiugas(numberwanted, verbose = true)
n, found, nfound = 6, Int[], 0
starttime = time()
while nfound < numberwanted
if n % 5 == 0 || n % 7 == 0 || n % 11 == 0
for (p, e) in eachfactor(n)
(e != 1 || rem(n ÷ p - 1, p) != 0) && @goto nextnumber
end
verbose && println(n, " (elapsed: ", time() - starttime, ")")
push!(found, n)
nfound += 1
end
@label nextnumber
n += 6 # all mult of 6
end
return found
end
 
@time getgiugas(2, false)
@time getgiugas(6)
 
Output:
30  (elapsed: 0.0)
858  (elapsed: 0.0)
1722  (elapsed: 0.0)
66198  (elapsed: 0.0009999275207519531)
2214408306  (elapsed: 18.97099995613098)
24423128562  (elapsed: 432.06500005722046)
432.066249 seconds (235 allocations: 12.523 KiB)

Pascal[edit]

Free Pascal[edit]

OK.Cheating to find square free numbers like julia in distance 6
That means always factors 2,3 and minimum one of 5,7,11.

program Giuga;
 
{$IFDEF FPC}
{$MODE DELPHI} {$OPTIMIZATION ON,ALL} {$COPERATORS ON}
{$ELSE}
{$APPTYPE CONSOLE}
{$ENDIF}
uses
sysutils
{$IFDEF WINDOWS},Windows{$ENDIF}
;
//######################################################################
//prime decomposition only squarefree and multiple of 6
 
type
tprimeFac = packed record
pfpotPrimIdx : array[0..9] of Uint64;
pfMaxIdx : Uint32;
end;
tpPrimeFac = ^tprimeFac;
tPrimes = array[0..65535] of Uint32;
 
var
{$ALIGN 8}
SmallPrimes: tPrimes;
{$ALIGN 32}
 
procedure InitSmallPrimes;
//get primes. #0..65535.Sieving only odd numbers
const
MAXLIMIT = (821641-1) shr 1;
var
pr : array[0..MAXLIMIT] of byte;
p,j,d,flipflop :NativeUInt;
Begin
SmallPrimes[0] := 2;
fillchar(pr[0],SizeOf(pr),#0);
p := 0;
repeat
repeat
p +=1
until pr[p]= 0;
j := (p+1)*p*2;
if j>MAXLIMIT then
BREAK;
d := 2*p+1;
repeat
pr[j] := 1;
j += d;
until j>MAXLIMIT;
until false;
 
SmallPrimes[1] := 3;
SmallPrimes[2] := 5;
j := 3;
d := 7;
flipflop := (2+1)-1;//7+2*2,11+2*1,13,17,19,23
p := 3;
repeat
if pr[p] = 0 then
begin
SmallPrimes[j] := d;
inc(j);
end;
d += 2*flipflop;
p+=flipflop;
flipflop := 3-flipflop;
until (p > MAXLIMIT) OR (j>High(SmallPrimes));
end;
 
function OutPots(pD:tpPrimeFac;n:NativeInt):Ansistring;
var
s: String[31];
chk,p: NativeInt;
Begin
str(n,s);
result := s+' :';
with pd^ do
begin
chk := 1;
For n := 0 to pfMaxIdx-1 do
Begin
if n>0 then
result += '*';
p := pfpotPrimIdx[n];
chk *= p;
str(p,s);
result += s;
end;
str(chk,s);
result += '_chk_'+s+'<';
end;
end;
 
function IsSquarefreeDecomp6(var res:tPrimeFac;n:Uint64):boolean;inline;
//factorize only not prime/semiprime and squarefree n= n div 6
var
pr,i,q,idx :NativeUInt;
Begin
with res do
Begin
Idx := 2;
 
q := n DIV 5;
if n = 5*q then
Begin
pfpotPrimIdx[2] := 5;
n := q;
q := q div 5;
if q*5=n then
EXIT(false);
inc(Idx);
end;
 
q := n DIV 7;
if n = 7*q then
Begin
pfpotPrimIdx[Idx] := 7;
n := q;
q := q div 7;
if q*7=n then
EXIT(false);
inc(Idx);
end;
 
q := n DIV 11;
if n = 11*q then
Begin
pfpotPrimIdx[Idx] := 11;
n := q;
q := q div 11;
if q*11=n then
EXIT(false);
inc(Idx);
end;
 
if Idx < 3 then
Exit(false);
 
i := 5;
while i < High(SmallPrimes) do
begin
pr := SmallPrimes[i];
q := n DIV pr;
//if n < pr*pr
if pr > q then
BREAK;
if n = pr*q then
Begin
pfpotPrimIdx[Idx] := pr;
n := q;
q := n div pr;
if pr*q = n then
EXIT(false);
inc(Idx);
end;
inc(i);
end;
if n <> 1 then
begin
pfpotPrimIdx[Idx] := n;
inc(Idx);
end;
pfMaxIdx := idx;
end;
exit(true);
end;
 
function ChkGiuga(n:Uint64;pPrimeDecomp :tpPrimeFac):boolean;inline;
var
p : Uint64;
idx: NativeInt;
begin
with pPrimeDecomp^ do
Begin
idx := pfMaxIdx-1;
repeat
p := pfpotPrimIdx[idx];
result := (((n DIV p)-1)MOD p) = 0;
if not(result) then
EXIT;
dec(idx);
until idx<0;
end;
end;
 
const
LMT = 24423128562;//2214408306;//
var
PrimeDecomp :tPrimeFac;
T0:Int64;
n,n6 : UInt64;
cnt:Uint32;
Begin
InitSmallPrimes;
 
T0 := GetTickCount64;
with PrimeDecomp do
begin
pfpotPrimIdx[0]:= 2;
pfpotPrimIdx[1]:= 3;
end;
n := 0;
n6 := 0;
cnt := 0;
repeat
//only multibles of 6
inc(n,6);
inc(n6);
//no square factor of 2
if n AND 3 = 0 then
continue;
//no square factor of 3
if n MOD 9 = 0 then
continue;
 
if IsSquarefreeDecomp6(PrimeDecomp,n6)then
if ChkGiuga(n,@PrimeDecomp) then
begin
inc(cnt);
writeln(cnt:3,'..',OutPots(@PrimeDecomp,n),' ',(GettickCount64-T0)/1000:6:3,' s');
end;
until n >= LMT;
T0 := GetTickCount64-T0;
writeln('Found ',cnt);
writeln('Tested til ',n,' runtime ',T0/1000:0:3,' s');
writeln;
writeln(OutPots(@PrimeDecomp,n));
end.
@home AMD 5600G ( 4,4 Ghz ) fpc3.2.2 -O4 -Xs:
  1..30 :2*3*5_chk_30<   0.000 s
  2..858 :2*3*11*13_chk_858<   0.000 s
  3..1722 :2*3*7*41_chk_1722<   0.000 s
  4..66198 :2*3*11*17*59_chk_66198<   0.000 s
  5..2214408306 :2*3*11*23*31*47057_chk_2214408306<  17.120 s 
  6..24423128562 :2*3*7*43*3041*4447_chk_24423128562<  450.180 s
Found 6
Tested til 24423128562 runtime 450.180 s

24423128562 :2*3*7*43*3041*4447_chk_24423128562
TIO.RUN (~2.3 Ghz )takes ~4x runtime ? ( 2214408306 DIV 2 ) in 36 secs :-(

alternative version[edit]

Generating recursive squarefree numbers of ascending primes and check those numbers.
2*3 are set fixed. 2*3*5 followed 2*3*7 than 2*3*11. Therefor the results are unsorted.

program Giuga;
{
30 = 2 * 3 * 5.
858 = 2 * 3 * 11 * 13.
1722 = 2 * 3 * 7 * 41.
66198 = 2 * 3 * 11 * 17 * 59.
2214408306 = 2 * 3 * 11 * 23 * 31 * 47057.
24423128562 = 2 * 3 * 7 * 43 * 3041 * 4447.
432749205173838 = 2 * 3 * 7 * 59 * 163 * 1381 * 775807.
14737133470010574 = 2 * 3 * 7 * 71 * 103 * 67213 * 713863.
550843391309130318 = 2 * 3 * 7 * 71 * 103 * 61559 * 29133437.
244197000982499715087866346 = 2 * 3 * 11 * 23 * 31 * 47137 * 28282147 * 3892535183.
554079914617070801288578559178 = 2 * 3 * 11 * 23 * 31 * 47059 * 2259696349 * 110725121051.
1910667181420507984555759916338506 = 2 * 3 * 7 * 43 * 1831 * 138683 * 2861051 * 1456230512169437. }

{$IFDEF FPC}
{$MODE DELPHI} {$OPTIMIZATION ON,ALL} {$COPERATORS ON}
{$ELSE}
{$APPTYPE CONSOLE}
{$ENDIF}
uses
sysutils
{$IFDEF WINDOWS},Windows{$ENDIF}
;
const
LMT =14737133470010574;// 432749205173838;//24423128562;//2214408306;
type
tFac = packed record
fMul :Uint64;
fPrime,
fPrimIdx,
fprimMaxIdx,dummy :Uint32;
dummy2: Uint64;
end;
tFacs = array[0..15] of tFac;
tPrimes = array[0..62157] of Uint32;//775807 see factor of 432749205173838
// tPrimes = array[0..4875{14379}] of Uint32;//sqrt 24423128562
// tPrimes = array[0..1807414] of Uint32;//29133437
// tPrimes = array[0..50847533] of Uint32;// 1e9
// tPrimes = array[0..5761454] of Uint32;//1e8
var
SmallPrimes: tPrimes;
T0 : Int64;
cnt:Uint32;
 
procedure InitSmallPrimes;
//get primes. #0..65535.Sieving only odd numbers
const
//MAXLIMIT = (trunc(sqrt(LMT)+1)-1) shr 1+4;
MAXLIMIT = 775807 DIV 2+1;//(trunc(sqrt(LMT)+1)-1) shr 1+4;
var
pr : array of byte;
pPr :pByte;
p,j,d,flipflop :NativeUInt;
Begin
SmallPrimes[0] := 2;
setlength(pr,MAXLIMIT);
pPr := @pr[0];
p := 0;
repeat
repeat
p +=1
until pPr[p]= 0;
j := (p+1)*p*2;
if j>MAXLIMIT then
BREAK;
d := 2*p+1;
repeat
pPr[j] := 1;
j += d;
until j>MAXLIMIT;
until false;
 
SmallPrimes[1] := 3;
SmallPrimes[2] := 5;
j := 3;
d := 7;
flipflop := (2+1)-1;//7+2*2,11+2*1,13,17,19,23
p := 3;
repeat
if pPr[p] = 0 then
begin
SmallPrimes[j] := d;
inc(j);
end;
d += 2*flipflop;
p+=flipflop;
flipflop := 3-flipflop;
until (p > MAXLIMIT) OR (j>High(SmallPrimes));
setlength(pr,0);
end;
 
procedure OutFac(var F:tFacs;maxIdx:Uint32);
var
i : integer;
begin
write(cnt:3,' ');
For i := 0 to maxIdx do
write(F[i].fPrime,'*');
write(#8,' = ',F[maxIdx].fMul);
writeln(' ',(GetTickCount64-T0)/1000:10:3,' s');
//readln;
end;
 
function ChkGiuga(var F:tFacs;MaxIdx:Uint32):boolean;inline;
var
n : Uint64;
idx: NativeInt;
p : Uint32;
begin
n := F[MaxIdx].fMul;
idx := MaxIdx;
repeat
p := F[idx].fPrime;
result := (((n DIV p)-1)MOD p) = 0;
if not(result) then
EXIT;
dec(idx);
until idx<0;
inc(cnt);
end;
 
procedure InsertNextPrimeFac(var F:tFacs;idx:Uint32);
var
Mul : Uint64;
i,p : uint32;
begin
with F[idx-1] do
begin
Mul := fMul;
i := fPrimIdx;
end;
 
while i<High(SmallPrimes) do
begin
inc(i);
with F[idx] do
begin
if i >fprimMaxIdx then
BREAK;
p := SmallPrimes[i];
if p*Mul>LMT then
BREAK;
fMul := p*Mul;
fPrime := p;
fPrimIdx := i;
IF (Mul-1) MOD p = 0 then
IF ChkGiuga(F,idx) then
OutFac(F,idx);
end;
// max 6 factors //for lmt 24e9 need 7 factors
if idx <5 then
InsertNextPrimeFac(F,idx+1);
end;
end;
 
var
{$ALIGN 32}
Facs : tFacs;
i : integer;
Begin
InitSmallPrimes;
 
T0 := GetTickCount64;
with Facs[0] do
begin
fMul := 2;fPrime := 2;fPrimIdx:= 0;
end;
with Facs[1] do
begin
fMul := 2*3;fPrime := 3;fPrimIdx:= 1;
end;
i := 2;
//search the indices of mx found factor
while SmallPrimes[i] < 11 do inc(i); Facs[2].fprimMaxIdx := i;
while SmallPrimes[i] < 71 do inc(i); Facs[3].fprimMaxIdx := i;
while SmallPrimes[i] < 3041 do inc(i); Facs[4].fprimMaxIdx := i;
while SmallPrimes[i] < 67213 do inc(i); Facs[5].fprimMaxIdx := i;
while SmallPrimes[i] < 775807 do inc(i); Facs[6].fprimMaxIdx := i;
{
writeln('Found ',cnt,' in ',(GetTickCount64-T0)/1000:10:3,' s');
//start with 2*3*7
with Facs[2] do
begin
fMul := 2*3*7;fPrime := 7;fPrimIdx:= 3;
end;
InsertNextPrimeFac(Facs,3);
//start with 2*3*11
writeln('Found ',cnt,' in ',(GetTickCount64-T0)/1000:10:3,' s');
with Facs[2] do
begin
fMul := 2*3*11;fPrime := 11;fPrimIdx:= 4;
end;
InsertNextPrimeFac(Facs,3);
}

InsertNextPrimeFac(Facs,2);
writeln('Found ',cnt,' in ',(GetTickCount64-T0)/1000:10:3,' s');
writeln;
end.
 
@TIO.RUN:
  1  2*3*5 = 30           0.000 s
  2  2*3*7*41 = 1722           0.810 s
  3  2*3*7*43*3041*4447 = 24423128562           0.871 s
  4  2*3*11*13 = 858           1.057 s
  5  2*3*11*17*59 = 66198           1.089 s
  6  2*3*11*23*31*47057 = 2214408306           1.152 s
Found 6 in      1.526 s
Real time: 1.682 s CPU share: 99.42 %

 @home: 
Limit:432749205173838
start 2*3*7
Found 0 in      0.000 s
  1  2*3*7*41 = 1722         147.206 s
  2  2*3*7*43*3041*4447 = 24423128562         163.765 s
  3  2*3*7*59*163*1381*775807 = 432749205173838         179.124 s
Found 3 in    197.002 s
start 2*3*11
  4  2*3*11*13 = 858         197.002 s
  5  2*3*11*17*59 = 66198         219.166 s
  6  2*3*11*23*31*47057 = 2214408306         244.468 s

real  5m10,271s

Limit :14737133470010574
start 2*3*7
Found 0 in      0.000 s
  1  2*3*7*41 = 1722        1330.819 s
  2  2*3*7*43*3041*4447 = 24423128562        1567.028 s
  3  2*3*7*59*163*1381*775807 = 432749205173838        1788.203 s
  4  2*3*7*71*103*67213*713863 = 14737133470010574        2051.552 s
Found 4 in   2129.801 s
start 2*3*11
  5  2*3*11*13 = 858        2129.801 s
  6  2*3*11*17*59 = 66198        2305.752 s
  7  2*3*11*23*31*47057 = 2214408306        2591.984 s
Found 7 in   3654.610 s

real  60m54,612s

Perl[edit]

#!/usr/bin/perl
 
use strict; # https://rosettacode.org/wiki/Giuga_numbers
use warnings;
use ntheory qw( factor forcomposites );
use List::Util qw( all );
 
forcomposites
{
my $n = $_;
all { ($n / $_ - 1) % $_ == 0 } factor $n and print "$n\n";
} 4, 67000;
Output:
30
858
1722
66198

Phix[edit]

with javascript_semantics
constant limit = 4
sequence giuga = {}
integer n = 4
while length(giuga)<limit do
    sequence pf = prime_factors(n)
    for f in pf do
        if remainder(n/f-1,f) then pf={} exit end if
    end for
    if length(pf) then giuga &= n end if
    n += 2
end while
printf(1,"The first %d Giuga numbers are: %v\n",{limit,giuga})
Output:
The first 4 Giuga numbers are: {30,858,1722,66198}

Faster version[edit]

Translation of: Wren
--
-- demo\rosetta\Giuga_number.exw
-- =============================
--
with javascript_semantics
requires("1.0.2") -- (is_prime2 tweak)

procedure giuga(integer n)
    printf(1,"n = %d:",n)
    sequence p = repeat(0,n),
             s = repeat(0,n)
    p[1..2] = 1
    s[1] = {1,2}
    integer t = 2
    while t>1 do
        integer pt = p[t]+1
        p[t] = pt
        pt = get_prime(pt)
        integer {c,d} = s[t-1]
        {c,d} = {c*pt+d,d*pt}
        s[t] = {c,d}
        if c=d
        or c*pt+(n-t)*d<=d*pt then
            t -= 1
        elsif t < (n - 2) then
            t += 1
            {c,d} = s[t-1]
            p[t] = max(p[t-1], is_prime2(floor(c/(d-c)),true))
        else
            {c,d} = s[n-2]
            integer dmc = d-c,
                    k = d*d-dmc
            sequence f = factors(k,1)
            for i=1 to floor(length(f)/2) do
                integer h = f[i]
                if remainder(h+d,dmc) == 0 
                and remainder(k/h+d,dmc) == 0 then
                    integer r1 = (h + d) / dmc,
                            r2 = (k/h + d) / dmc,
                            pn2 = get_prime(p[n-2])
                    if  r1 > pn2 
                    and r2 > pn2 
                    and r1 != r2 
                    and is_prime(r1) 
                    and is_prime(r2) then
                        printf(1," %d",d * r1 * r2)
                    end if
                end if
            end for
        end if
    end while
    printf(1,"\n")
end procedure
 
papply({3,4,5,6},giuga)
Output:
n = 3: 30
n = 4: 1722 858
n = 5: 66198
n = 6: 24423128562 2214408306

You can (almost) push things a little further on 64-bit:

if machine_bits()=64 then giuga(7) end if

and get

n = 7: 432749205173838 550843391309130318 14737133470010574

It took about 3 minutes for those to show, but then carried on in a doomed quest for an hour before I killed it.

Python[edit]

Translation of: FreeBASIC
#!/usr/bin/python
 
from math import sqrt
 
def isGiuga(m):
n = m
f = 2
l = sqrt(n)
while True:
if n % f == 0:
if ((m / f) - 1) % f != 0:
return False
n /= f
if f > n:
return True
else:
f += 1
if f > l:
return False
 
 
if __name__ == '__main__':
n = 3
c = 0
print("The first 4 Giuga numbers are: ")
while c < 4:
if isGiuga(n):
c += 1
print(n)
n += 1


Raku[edit]

my @primes = (3..60).grep: &is-prime;
 
print 'First four Giuga numbers: ';
 
put sort flat (2..4).map: -> $c {
@primes.combinations($c).map: {
my $n = [×] 2,|$_;
$n if all .map: { ($n / $_ - 1) %% $_ };
}
}
Output:
First 4 Giuga numbers: 30 858 1722 66198

Wren[edit]

Version 1 (Brute force)[edit]

Simple brute force but assumes all Giuga numbers will be even, must be square-free and can't be semi-prime.

Takes only about 0.05 seconds to find the first four Giuga numbers but finding the fifth would take many hours using this approach, so I haven't bothered.

var factors = []
var inc = [4, 2, 4, 2, 4, 6, 2, 6]
 
// Assumes n is even with exactly one factor of 2.
// Empties 'factors' if any other prime factor is repeated.
var primeFactors = Fn.new { |n|
factors.clear()
var last = 2
factors.add(2)
n = (n/2).truncate
while (n%3 == 0) {
if (last == 3) {
factors.clear()
return
}
last = 3
factors.add(3)
n = (n/3).truncate
}
while (n%5 == 0) {
if (last == 5) {
factors.clear()
return
}
last = 5
factors.add(5)
n = (n/5).truncate
}
var k = 7
var i = 0
while (k * k <= n) {
if (n%k == 0) {
if (last == k) {
factors.clear()
return
}
last = k
factors.add(k)
n = (n/k).truncate
} else {
k = k + inc[i]
i = (i + 1) % 8
}
}
if (n > 1) factors.add(n)
}
 
var limit = 4
var giuga = []
var n = 6 // can't be 2 or 4
while (giuga.count < limit) {
primeFactors.call(n)
// can't be prime or semi-prime
if (factors.count > 2 && factors.all { |f| (n/f - 1) % f == 0 }) {
giuga.add(n)
}
n = n + 4 // can't be divisible by 4
}
System.print("The first %(limit) Giuga numbers are:")
System.print(giuga)
Output:
The first 4 Giuga numbers are:
[30, 858, 1722, 66198]


Version 2 (Pari-GP translation)[edit]

Library: Wren-math
Library: Wren-rat

This is a translation of the very fast Pari-GP code in the talk page. Only takes 0.015 seconds to find the first six Giuga numbers.

import "./math" for Math, Int
import "./rat" for Rat
 
var giuga = Fn.new { |n|
System.print("n = %(n):")
var p = List.filled(n, 0)
var s = List.filled(n, null)
for (i in 0..n-2) s[i] = Rat.zero
p[2] = 2
p[1] = 2
var t = 2
s[1] = Rat.half
while (t > 1) {
p[t] = Int.isPrime(p[t] + 1) ? p[t] + 1 : Int.nextPrime(p[t] + 1)
s[t] = s[t-1] + Rat.new(1, p[t])
if (s[t] == Rat.one || s[t] + Rat.new(n - t, p[t]) <= Rat.one) {
t = t - 1
} else if (t < n - 2) {
t = t + 1
p[t] = Math.max(p[t-1], (s[t-1] / (Rat.one - s[t-1])).toFloat).floor
} else {
var c = s[n-2].num
var d = s[n-2].den
var k = d * d + c - d
var f = Int.divisors(k)
for (i in 0...((f.count + 1)/2).floor) {
var h = f[i]
if ((h + d) % (d-c) == 0 && (k/h + d) % (d - c) == 0) {
var r1 = (h + d) / (d - c)
var r2 = (k/h + d) / (d - c)
if (r1 > p[n-2] && r2 > p[n-2] && r1 != r2 && Int.isPrime(r1) && Int.isPrime(r2)) {
var w = d * r1 * r2
System.print(w)
}
}
}
}
}
}
 
for (n in 3..6) {
giuga.call(n)
System.print()
}
Output:
n = 3:
30

n = 4:
1722
858

n = 5:
66198

n = 6:
24423128562
2214408306

XPL0[edit]

func Giuga(N0);         \Return 'true' if Giuga number
int N0;
int N, F, Q1, Q2, L;
[N:= N0; F:= 2; L:= sqrt(N);
loop [Q1:= N/F;
if rem(0) = 0 then \found a prime factor
[Q2:= N0/F;
if rem((Q2-1)/F) # 0 then return false;
N:= Q1;
if F>N then quit;
]
else [F:= F+1;
if F>L then return false;
];
];
return true;
];
 
int N, C;
[N:= 3; C:= 0;
loop [if Giuga(N) then
[IntOut(0, N); ChOut(0, ^ );
C:= C+1;
if C >= 4 then quit;
];
N:= N+1;
];
]
Output:
30 858 1722 66198