Sequence: smallest number with exactly n divisors
Calculate the sequence where each term an is the smallest natural number that has exactly n divisors.
- Task
Show here, on this page, at least the first 15 terms of the sequence.
- See also
- Related tasks
ALGOL 68
<lang algol68>BEGIN
PROC count divisors = ( INT n )INT:
BEGIN
INT count := 0;
FOR i WHILE i*i <= n DO
IF n MOD i = 0 THEN
count +:= IF i = n OVER i THEN 1 ELSE 2 FI
FI
OD;
count
END # count divisors # ;
INT max = 15;
[ max ]INT seq;FOR i TO max DO seq[ i ] := 0 OD;
INT found := 0;
FOR i WHILE found < max DO
IF INT divisors = count divisors( i );
divisors <= max
THEN
# have an i with an appropriate number of divisors #
IF seq[ divisors ] = 0 THEN
# this is the first i with that many divisors #
seq[ divisors ] := i;
found +:= 1
FI
FI
OD;
print( ( "The first ", whole( max, 0 ), " terms of the sequence are:", newline ) );
FOR i TO max DO
print( ( whole( seq( i ), 0 ), " " ) )
OD;
print( ( newline ) )
END</lang>
- Output:
The first 15 terms of the sequence are: 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144
C
<lang c>#include <stdio.h>
- define MAX 15
int count_divisors(int n) {
int i, count = 0;
for (i = 1; i * i <= n; ++i) {
if (!(n % i)) {
if (i == n / i)
count++;
else
count += 2;
}
}
return count;
}
int main() {
int i, k, n, seq[MAX];
for (i = 0; i < MAX; ++i) seq[i] = 0;
printf("The first %d terms of the sequence are:\n", MAX);
for (i = 1, n = 0; n < MAX; ++i) {
k = count_divisors(i);
if (k <= MAX && seq[k - 1] == 0) {
seq[k - 1] = i;
++n;
}
}
for (i = 0; i < MAX; ++i) printf("%d ", seq[i]);
printf("\n");
return 0;
}</lang>
- Output:
The first 15 terms of the sequence are: 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144
C++
<lang cpp>#include <iostream>
- define MAX 15
using namespace std;
int count_divisors(int n) {
int count = 0;
for (int i = 1; i * i <= n; ++i) {
if (!(n % i)) {
if (i == n / i)
count++;
else
count += 2;
}
}
return count;
}
int main() {
int i, k, n, seq[MAX];
for (i = 0; i < MAX; ++i) seq[i] = 0;
cout << "The first " << MAX << " terms of the sequence are:" << endl;
for (i = 1, n = 0; n < MAX; ++i) {
k = count_divisors(i);
if (k <= MAX && seq[k - 1] == 0) {
seq[k - 1] = i;
++n;
}
}
for (i = 0; i < MAX; ++i) cout << seq[i] << " ";
cout << endl;
return 0;
}</lang>
- Output:
The first 15 terms of the sequence are: 1 2 4 6 16 12 64 24 36 48 1024 60 0 192 144
F#
This task uses Extensible Prime Generator (F#) <lang fsharp> // Find Antı-Primes plus. Nigel Galloway: April 9th., 2019 // Increasing the value 14 will increase the number of anti-primes plus found let fI=primes|>Seq.take 14|>Seq.map bigint|>List.ofSeq let N=Seq.reduce(*) fI let fG g=Seq.unfold(fun ((n,i,e) as z)->Some(z,(n+1,i+1,(e*g)))) (1,2,g) let fE n i=n|>Seq.collect(fun(n,e,g)->Seq.map(fun(a,c,b)->(a,c*e,g*b)) (i|>Seq.takeWhile(fun(g,_,_)->g<=n))|> Seq.takeWhile(fun(_,_,n)->n<N)) let fL=let mutable g=0 in (fun n->g<-g+1; n=g) let n=Seq.concat(Seq.scan(fun n g->fE n (fG g)) (seq[(2147483647,1,1I)]) fI)|>List.ofSeq|>List.groupBy(fun(_,n,_)->n)|>List.sortBy(fun(n,_)->n)|>List.takeWhile(fun(n,_)->fL n) for n,g in n do printfn "%d->%A" n (g|>List.map(fun(_,_,n)->n)|>List.min) </lang>
- Output:
1->1 2->2 3->4 4->6 5->16 6->12 7->64 8->24 9->36 10->48 11->1024 12->60 13->4096 14->192 15->144 16->120 17->65536 18->180 19->262144 20->240 21->576 22->3072 23->4194304 24->360 25->1296 26->12288 27->900 28->960 29->268435456 30->720 31->1073741824 32->840 33->9216 34->196608 35->5184 36->1260 37->68719476736 38->786432 39->36864 40->1680 41->1099511627776 42->2880 43->4398046511104 44->15360 45->3600 46->12582912 47->70368744177664 48->2520 49->46656 50->6480 51->589824 52->61440 53->4503599627370496 54->6300 55->82944 56->6720 57->2359296 58->805306368 Real: 00:00:01.079, CPU: 00:00:01.080, GC gen0: 47, gen1: 0
Factor
<lang factor>USING: fry kernel lists lists.lazy math math.primes.factors prettyprint sequences ;
- A005179 ( -- list )
1 lfrom [
1 swap '[ dup divisors length _ = ] [ 1 + ] until
] lmap-lazy ;
15 A005179 ltake list>array .</lang>
- Output:
{ 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144 }
Go
<lang go>package main
import "fmt"
func countDivisors(n int) int {
count := 0
for i := 1; i*i <= n; i++ {
if n%i == 0 {
if i == n/i {
count++
} else {
count += 2
}
}
}
return count
}
func main() {
const max = 15
seq := make([]int, max)
fmt.Println("The first", max, "terms of the sequence are:")
for i, n := 1, 0; n < max; i++ {
if k := countDivisors(i); k <= max && seq[k-1] == 0 {
seq[k-1] = i
n++
}
}
fmt.Println(seq)
}</lang>
- Output:
The first 15 terms of the sequence are: [1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144]
Java
<lang java>import java.util.Arrays;
public class OEIS_A005179 {
static int count_divisors(int n) {
int count = 0;
for (int i = 1; i * i <= n; ++i) {
if (n % i == 0) {
if (i == n / i)
count++;
else
count += 2;
}
}
return count;
}
public static void main(String[] args) {
final int max = 15;
int[] seq = new int[max];
System.out.printf("The first %d terms of the sequence are:\n", max);
for (int i = 1, n = 0; n < max; ++i) {
int k = count_divisors(i);
if (k <= max && seq[k - 1] == 0) {
seq[k- 1] = i;
n++;
}
}
System.out.println(Arrays.toString(seq));
}
}</lang>
- Output:
The first 15 terms of the sequence are: [1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]
Kotlin
<lang scala>// Version 1.3.21
const val MAX = 15
fun countDivisors(n: Int): Int {
var count = 0
var i = 1
while (i * i <= n) {
if (n % i == 0) {
count += if (i == n / i) 1 else 2
}
i++
}
return count
}
fun main() {
var seq = IntArray(MAX)
println("The first $MAX terms of the sequence are:")
var i = 1
var n = 0
while (n < MAX) {
var k = countDivisors(i)
if (k <= MAX && seq[k - 1] == 0) {
seq[k - 1] = i
n++
}
i++
}
println(seq.asList())
}</lang>
- Output:
The first 15 terms of the sequence are: [1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]
Perl
<lang perl>use strict; use warnings; use ntheory 'divisors';
print "First 15 terms of OEIS: A005179\n"; for my $n (1..15) {
my $l = 0;
while (++$l) {
print "$l " and last if $n == divisors($l);
}
}</lang>
- Output:
First 15 terms of OEIS: A005179 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144
Perl 6
<lang perl6>sub div-count (\x) {
return 2 if x.is-prime;
+flat (1 .. x.sqrt.floor).map: -> \d {
unless x % d { my \y = x div d; y == d ?? y !! (y, d) }
}
}
my $limit = 15;
put "First $limit terms of OEIS:A005179"; put (1..$limit).map: -> $n { first { $n == .&div-count }, 1..Inf };
</lang>
- Output:
First 15 terms of OEIS:A005179 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144
Phix
<lang Phix>constant limit = 15 sequence res = repeat(0,limit) integer found = 0, n = 1 while found<limit do
integer k = length(factors(n,1))
if k<=limit and res[k]=0 then
res[k] = n
found += 1
end if
n += 1
end while printf(1,"The first %d terms are: %v\n",{limit,res})</lang>
- Output:
The first 15 terms are: {1,2,4,6,16,12,64,24,36,48,1024,60,4096,192,144}
You would need something quite a bit smarter to venture over a limit of 28.
REXX
<lang rexx>/*REXX program finds and displays the smallest number with exactly N divisors.*/ parse arg N . /*obtain optional argument from the CL.*/ if N== | N=="," then N= 15 /*Not specified? Then use the default.*/ say '──divisors── ──smallest number with N divisors──' /*display title for the numbers.*/ @.= /*the @ array is used for memoization*/
do i=1 for N /*step through a number of divisors. */
do j=1+(i\==1) by 1+(i\==1) /*now, search for a number that ≡ #divs*/
if @.j==. then iterate /*has this number already been found? */
d= #divs(j); if d\==i then iterate /*get # divisors; Is not equal? Skip.*/
say center(i, 12) right(j, 19) /*display the #divs and the smallest #.*/
@.j=. /*mark as having found #divs for this J*/
leave /*found a number, so now get the next I*/
end /*j*/
end /*i*/
exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/
- divs: procedure; parse arg x 1 y /*X and Y: both set from 1st argument.*/
if x<7 then do /*handle special cases for numbers < 7.*/
if x<3 then return x /* " " " " one and two.*/
if x<5 then return x - 1 /* " " " " three & four*/
if x==5 then return 2 /* " " " " five. */
if x==6 then return 4 /* " " " " six. */
end
odd= x // 2 /*check if X is odd or not. */
if odd then do; #= 1; end /*Odd? Assume Pdivisors count of 1.*/
else do; #= 3; y= x%2; end /*Even? " " " " 3.*/
/* [↑] start with known num of Pdivs.*/
do k=3 for x%2-3 by 1+odd while k<y /*for odd numbers, skip evens.*/
if x//k==0 then do /*if no remainder, then found a divisor*/
#=#+2; y=x%k /*bump # Pdivs, calculate limit Y. */
if k>=y then do; #= #-1; leave; end /*limit?*/
end /* ___ */
else if k*k>x then leave /*only divide up to √ x */
end /*k*/ /* [↑] this form of DO loop is faster.*/
return #+1 /*bump "proper divisors" to "divisors".*/</lang>
- output when using the default input:
──divisors── ──smallest number with N divisors──
1 1
2 2
3 4
4 6
5 16
6 12
7 64
8 24
9 36
10 48
11 1024
12 60
13 4096
14 192
15 144
Sidef
<lang ruby>func n_divisors(n) {
1..Inf -> first_by { .sigma0 == n }
}
say 15.of { n_divisors(_+1) }</lang>
- Output:
[1, 2, 4, 6, 16, 12, 64, 24, 36, 48, 1024, 60, 4096, 192, 144]
zkl
<lang zkl>fcn countDivisors(n)
{ [1.. n.toFloat().sqrt()].reduce('wrap(s,i){ s + (if(0==n%i) 1 + (i!=n/i)) },0) }
A005179w:=(1).walker(*).tweak(fcn(n){
var N=0,cache=Dictionary();
if(cache.find(n)) return(cache.pop(n)); // prune
while(1){
if(n == (d:=countDivisors(N+=1))) return(N);
if(n<d and not cache.find(d)) cache[d]=N;
}
});</lang> <lang zkl>N:=15; println("First %d terms of OEIS:A005179".fmt(N)); A005179w.walk(N).concat(" ").println();</lang>
- Output:
First 15 terms of OEIS:A005179 1 2 4 6 16 12 64 24 36 48 1024 60 4096 192 144