Circular primes
- Definitions
A circular prime is a prime number with the property that the number generated at each intermediate step when cyclically permuting its (base 10) digits will also be prime.
For example: 1193 is a circular prime, since 1931, 9311 and 3119 are all also prime.
Note that a number which is a cyclic permutation of a smaller circular prime is not considered to be itself a circular prime. So 13 is a circular prime but 31 is not.
A repunit (denoted by R) is a number whose base 10 representation contains only the digit 1.
For example: R(2) = 11 and R(5) = 11111 are repunits.
- Task
- Find the first 19 circular primes.
- If your language has access to arbitrary precision integer arithmetic, given that they are all repunits, find the next 4 circular primes.
- (Stretch) Determine which of the following repunits are probably circular primes: R(5003), R(9887), R(15073), R(25031), R(35317) and R(49081). The larger ones may take a long time to process so just do as many as you reasonably can.
- See also
- Wikipedia article - Circular primes.
- Wikipedia article - Repunit.
- OEIS sequence A016114 - Circular primes.
Factor
Unfortunately Factor's miller-rabin test or bignums aren't quite up to the task of finding the next four circular prime repunits in a reasonable time. It takes ~90 seconds to check R(7)-R(1031).
<lang factor>USING: combinators.short-circuit formatting io kernel lists lists.lazy math math.combinatorics math.functions math.parser math.primes sequences sequences.extras ;
! Create an ordered infinite lazy list of circular prime ! "candidates" -- the numbers 2, 3, 5 followed by numbers ! composed of only the digits 1, 3, 7, and 9.
- candidates ( -- list )
L{ "2" "3" "5" "7" } 2 lfrom [ "1379" swap selections >list ] lmap-lazy lconcat lappend ;
- circular-prime? ( str -- ? )
all-rotations { [ [ infimum ] [ first = ] bi ] [ [ string>number prime? ] all? ] } 1&& ;
- circular-primes ( -- list )
candidates [ circular-prime? ] lfilter ;
- prime-repunits ( -- list )
7 lfrom [ 10^ 1 - 9 / prime? ] lfilter ;
"The first 19 circular primes are:" print 19 circular-primes ltake [ write bl ] leach nl nl
"The next 4 circular primes, in repunit format, are:" print 4 prime-repunits ltake [ "R(%d) " printf ] leach nl</lang>
- Output:
The first 19 circular primes are: 2 3 5 7 11 13 17 37 79 113 197 199 337 1193 3779 11939 19937 193939 199933 The next 4 circular primes, in repunit format, are: R(19) R(23) R(317) R(1031)
Go
<lang go>package main
import (
"fmt" big "github.com/ncw/gmp" "strings"
)
// OK for 'small' numbers. func isPrime(n int) bool {
switch { case n < 2: return false case n%2 == 0: return n == 2 case n%3 == 0: return n == 3 default: d := 5 for d*d <= n { if n%d == 0 { return false } d += 2 if n%d == 0 { return false } d += 4 } return true }
}
func repunit(n int) *big.Int {
ones := strings.Repeat("1", n) b, _ := new(big.Int).SetString(ones, 10) return b
}
var circs = []int{}
// binary search is overkill for a small number of elements func alreadyFound(n int) bool {
for _, i := range circs { if i == n { return true } } return false
}
func isCircular(n int) bool {
nn := n pow := 1 // will eventually contain 10 ^ d where d is number of digits in n for nn > 0 { pow *= 10 nn /= 10 } nn = n for { nn *= 10 f := nn / pow // first digit nn += f * (1 - pow) if alreadyFound(nn) { return false } if nn == n { break } if !isPrime(nn) { return false } } return true
}
func main() {
fmt.Println("The first 19 circular primes are:") digits := [4]int{1, 3, 7, 9} q := []int{1, 2, 3, 5, 7, 9} // queue the numbers to be examined fq := []int{1, 2, 3, 5, 7, 9} // also queue the corresponding first digits count := 0 for { f := q[0] // peek first element fd := fq[0] // peek first digit if isPrime(f) && isCircular(f) { circs = append(circs, f) count++ if count == 19 { break } } copy(q, q[1:]) // pop first element q = q[:len(q)-1] // reduce length by 1 copy(fq, fq[1:]) // ditto for first digit queue fq = fq[:len(fq)-1] if f == 2 || f == 5 { // if digits > 1 can't contain a 2 or 5 continue } // add numbers with one more digit to queue // only numbers whose last digit >= first digit need be added for _, d := range digits { if d >= fd { q = append(q, f*10+d) fq = append(fq, fd) } } } fmt.Println(circs) fmt.Println("\nThe next 4 circular primes, in repunit format, are:") count = 0 var rus []string for i := 7; count < 4; i++ { if repunit(i).ProbablyPrime(10) { count++ rus = append(rus, fmt.Sprintf("R(%d)", i)) } } fmt.Println(rus) fmt.Println("\nThe following repunits are probably circular primes:") for _, i := range []int{5003, 9887, 15073, 25031, 35317, 49081} { fmt.Printf("R(%-5d) : %t\n", i, repunit(i).ProbablyPrime(10)) }
}</lang>
- Output:
The first 19 circular primes are: [2 3 5 7 11 13 17 37 79 113 197 199 337 1193 3779 11939 19937 193939 199933] The next 4 circular primes, in repunit format, are: [R(19) R(23) R(317) R(1031)] The following repunits are probably circular primes: R(5003 ) : false R(9887 ) : false R(15073) : false R(25031) : false R(35317) : false R(49081) : true
Julia
Note that the evalpoly function used in this program was added in Julia 1.4 <lang julia>using Lazy, Primes
function iscircularprime(n)
!isprime(n) && return false dig = digits(n) return all(i -> (m = evalpoly(10, circshift(dig, i))) >= n && isprime(m), 1:length(dig)-1)
end
filtcircular(n, rang) = Int.(collect(take(n, filter(iscircularprime, rang)))) isprimerepunit(n) = isprime(evalpoly(BigInt(10), ones(Int, n))) filtrep(n, rang) = collect(take(n, filter(isprimerepunit, rang)))
println("The first 19 circular primes are:\n", filtcircular(19, Lazy.range(2))) print("\nThe next 4 circular primes, in repunit format, are: ",
mapreduce(n -> "R($n) ", *, filtrep(4, Lazy.range(6))))
println("\n\nChecking larger repunits:") for i in [5003, 9887, 15073, 25031, 35317, 49081]
println("R($i) is ", isprimerepunit(i) ? "prime." : "not prime.")
end
</lang>
- Output:
The first 19 circular primes are: [2, 3, 5, 7, 11, 13, 17, 37, 79, 113, 197, 199, 337, 1193, 3779, 11939, 19937, 193939, 199933] The next 4 circular primes, in repunit format, are: R(19) R(23) R(317) R(1031) Checking larger repunits: R(5003) is not prime. R(9887) is not prime. R(15073) is not prime. R(25031) is not prime. R(35317) is not prime. R(49081) is prime.
Perl
<lang perl>use strict; use warnings; use feature 'say'; use List::Util 'min'; use ntheory 'is_prime';
sub rotate { my($i,@a) = @_; join , @a[$i .. @a-1, 0 .. $i-1] }
sub isCircular {
my ($n) = @_; return 0 unless is_prime($n); my @circular = split //, $n; return 0 if min(@circular) < $circular[0]; for (1 .. scalar @circular) { my $r = join , rotate($_,@circular); return 0 unless is_prime($r) and $r >= $n; } 1
}
say "The first 19 circular primes are:"; for ( my $i = 1, my $count = 0; $count < 19; $i++ ) {
++$count and print "$i " if isCircular($i);
}
say "\n\nThe next 4 circular primes, in repunit format, are:"; for ( my $i = 7, my $count = 0; $count < 4; $i++ ) {
++$count and say "R($i)" if is_prime 1 x $i
}
say "\nRepunit testing:";
for (5003, 9887, 15073, 25031, 35317, 49081) {
say "R($_): Prime? " . (is_prime 1 x $_ ? 'True' : 'False');
}</lang>
- Output:
The first 19 circular primes are: 2 3 5 7 11 13 17 37 79 113 197 199 337 1193 3779 11939 19937 193939 199933 The next 4 circular primes, in repunit format, are: R(19) R(23) R(317) R(1031) Repunit testing: R(5003): Prime? False R(9887): Prime? False R(15073): Prime? False R(25031): Prime? False R(35317): Prime? False R(49081): Prime? True
Phix
<lang Phix>function circular(integer p)
integer len = length(sprintf("%d",p)), pow = power(10,len-1), p0 = p for i=1 to len-1 do p = pow*remainder(p,10)+floor(p/10) if p<p0 or not is_prime(p) then return false end if end for return true
end function
sequence c = {} integer n = 1 while length(c)<19 do
integer p = get_prime(n) if circular(p) then c &= p end if n += 1
end while printf(1,"The first 19 circular primes are:\n%v\n\n",{c})
include mpfr.e procedure repunit(mpz z, integer n)
mpz_set_str(z,repeat('1',n))
end procedure
c = {} n = 7 mpz z = mpz_init() randstate state = gmp_randinit_mt() while length(c)<4 do
repunit(z,n) if mpz_probable_prime_p(z,state) then c = append(c,sprintf("R(%d)",n)) end if n += 1
end while printf(1,"The next 4 circular primes, in repunit format, are:\n%s\n\n",{join(c)})</lang>
- Output:
The first 19 circular primes are: {2,3,5,7,11,13,17,37,79,113,197,199,337,1193,3779,11939,19937,193939,199933} The next 4 circular primes, in repunit format, are: R(19) R(23) R(317) R(1031)
stretch
<lang Phix>constant t = {5003, 9887, 15073, 25031, 35317, 49081} printf(1,"The following repunits are probably circular primes:\n") for i=1 to length(t) do
integer ti = t[i] atom t0 = time() repunit(z,ti) bool bPrime = mpz_probable_prime_p(z,state,1) printf(1,"R(%d) : %t (%s)\n", {ti, bPrime, elapsed(time()-t0)})
end for</lang>
- Output:
64-bit can only cope with the first five (it terminates abruptly on the sixth)
For comparison, the above Julia code (8/4/20, 64 bit) manages 1s, 5.6s, 15s, 50s, 1 min 50s (and 1 hour 45 min 40s) on the same box.
And Perl (somehow) manages 0/0/0/55s/0/21 mins 35 secs...
The following repunits are probably circular primes: R(5003) : false (2.0s) R(9887) : false (13.5s) R(15073) : false (45.9s) R(25031) : false (1 minute and 19s) R(35317) : false (3 minutes and 04s)
32-bit is much slower and can only cope with the first four
The following repunits are probably circular primes: R(5003) : false (10.2s) R(9887) : false (54.9s) R(15073) : false (2 minutes and 22s) R(25031) : false (7 minutes and 45s) diag looping, error code is 1, era is #00644651
Raku
Most of the repunit testing is relatively speedy using the ntheory library. The really slow ones are R(25031), at ~42 seconds and R(49081) at 922(!!) seconds.
<lang perl6>#!/usr/bin/env raku
- 20200406 Raku programming solution
sub isCircular(\n) {
return False unless n.is-prime; my @circular = n.comb; return False if @circular.min < @circular[0]; for 1 ..^ @circular -> $i { return False unless .is-prime and $_ >= n given @circular.rotate($i).join; } True
}
say "The first 19 circular primes are:"; say ((2..*).hyper.grep: { isCircular $_ })[^19];
say "\nThe next 4 circular primes, in repunit format, are:"; loop ( my $i = 7, my $count = 0; $count < 4; $i++ ) {
++$count, say "R($i)" if (1 x $i).is-prime
}
use ntheory:from<Perl5> qw[is_prime];
say "\nRepunit testing:";
(5003, 9887, 15073, 25031, 35317, 49081).map: {
my $now = now; say "R($_): Prime? ", ?is_prime("{1 x $_}"), " {(now - $now).fmt: '%.2f'}"
}</lang>
- Output:
The first 19 circular primes are: (2 3 5 7 11 13 17 37 79 113 197 199 337 1193 3779 11939 19937 193939 199933) The next 4 circular primes, in repunit format, are: R(19) R(23) R(317) R(1031) Repunit testing: R(5003): Prime? False 0.00 R(9887): Prime? False 0.01 R(15073): Prime? False 0.02 R(25031): Prime? False 41.40 R(35317): Prime? False 0.32 R(49081): Prime? True 921.73
REXX
By far, the greatest cost of CPU time is in the generation of a suitable number of primes. <lang rexx>/*REXX program finds & displays circular primes (with a title & in a horizontal format).*/ parse arg N hp . /*obtain optional arguments from the CL*/ if N== | N=="," then N= 19 /* " " " " " " */ if hp== | hp=="," then hp= 1000000 /* " " " " " " */ call genP /*gen primes up to hp (200,000). */ q= 024568 /*digs that most circular P can't have.*/
- =0; $= /*#: circular P count; $: is a list.*/
do j=2 until #==N /* [↓] traipse through the number(s).*/ if \!.j then iterate /*Is J not prime? Then skip number.*/ if j>9 & verify(j, q, 'M')>0 then iterate /*Does J contain forbidden digs? Skip.*/ if \circP(j) then iterate /*Not circular? Then skip this number.*/ #= # + 1 /*bump the count of circular primes. */ $= $ j /*add this prime number ──► $ list. */ end /*j*/ /*at this point, $ has a leading blank.*/
say center(' first' # "circular primes ", 79, '─') say strip($) exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ circP: procedure expose @. !.; parse arg x 1 ox /*obtain a prime number to be examined.*/
do length(x)-1 /*parse X number, rotating the digits*/ parse var x 2 y 1 f 2 /*get rightmost digs & the first digit.*/ x= y || f /*construct a new possible circular P. */ if x<ox then return 0 /*is number < the original? ¬ circular*/ if \!.x then return 0 /* " " not prime? ¬ circular*/ end /*len···*/ return 1 /*passed all tests, X is a circular P.*/
/*──────────────────────────────────────────────────────────────────────────────────────*/ genP: @.1=2; @.2=3; @.3=5; @.4=7; @.5=11; @.6= 13; nP=6 /*assign low primes; # primes. */
!.= 0; !.2=1; !.3=1; !.5=1; !.7=1; !.11=1; !.13=1 /*assign primality to numbers. */ do j=@.nP+4 by 2 to hp /*only find odd primes from here on. */ if j// 3==0 then iterate /*is J divisible by #3 Then not prime*/ parse var j -1 _;if _==5 then iterate /*Is last digit a "5"? " " " */ if j// 7==0 then iterate /*is J divisible by 7? " " " */ if j//11==0 then iterate /* " " " " 11? " " " */ if j//13==0 then iterate /*is " " " 13? " " " */ do k=7 while k*k<=j /*divide by some generated odd primes. */ if j // @.k==0 then iterate j /*Is J divisible by P? Then not prime*/ end /*k*/ /* [↓] a prime (J) has been found. */ nP= nP+1; !.j=1; @.nP= j /*bump P cnt; assign P to @. and !. */ end /*j*/; return</lang>
- output when using the default inputs:
────────────────────────── first 19 circular primes ─────────────────────────── 2 3 5 7 11 13 17 37 79 113 197 199 337 1193 3779 11939 19937 193939 199933
Wren
Just the first part as no 'big integer' support. <lang ecmascript>var isPrime = Fn.new { |n|
if (n < 2 || !n.isInteger) return false if (n%2 == 0) return n == 2 if (n%3 == 0) return n == 3 var d = 5 while (d*d <= n) { if (n%d == 0) return false d = d + 2 if (n%d == 0) return false d = d + 4 } return true
}
var circs = []
var isCircular = Fn.new { |n|
var nn = n var pow = 1 // will eventually contain 10 ^ d where d is number of digits in n while (nn > 0) { pow = pow * 10 nn = (nn/10).floor } nn = n while (true) { nn = nn * 10 var f = (nn/pow).floor // first digit nn = nn + f * (1 - pow) if (circs.contains(nn)) return false if (nn == n) break if (!isPrime.call(nn)) return false } return true
}
System.print("The first 19 circular primes are:") var digits = [1, 3, 7, 9] var q = [1, 2, 3, 5, 7, 9] // queue the numbers to be examined var fq = [1, 2, 3, 5, 7, 9] // also queue the corresponding first digits var count = 0 while (true) {
var f = q[0] // peek first element var fd = fq[0] // peek first digit if (isPrime.call(f) && isCircular.call(f)) { circs.add(f) count = count + 1 if (count == 19) break } q.removeAt(0) // pop first element fq.removeAt(0) // ditto for first digit queue if (f != 2 && f != 5) { // if digits > 1 can't contain a 2 or 5 // add numbers with one more digit to queue // only numbers whose last digit >= first digit need be added for (d in digits) { if (d >= fd) { q.add(f*10+d) fq.add(fd) } } }
} System.print(circs)</lang>
- Output:
The first 19 circular primes are: [2, 3, 5, 7, 11, 13, 17, 37, 79, 113, 197, 199, 337, 1193, 3779, 11939, 19937, 193939, 199933]