## A Sobering Problem

Anything goes, but keep it seemly...

### Re: A Sobering Problem

Smythe Dakota wrote:Let's do it with numbers. Somebody come up with two 5-digit prime numbers, and ask for a path between them, changing one digit at a time, such that every intermediate 5-digit number is prime.

Bill Smythe

As far as I can tell there are 8363 five digit prime numbers ranging from 10007 - 99991. Any two such primes can be connected via a minimum path of at most 10 steps (from the first prime). There is only 1 pair of five digit primes, 88259 and 99721 where a 10 step connection path is required, one such path being:

Code: Select all
`   88259 1 78259 2 79259 3 79159 4 79151 5 79181 6 39181 7 39161 8 39761 9 9976110 99721`

A minimum connection path between the lowest and highest 5 digit primes (10007 and 99991) requires only 5 steps, one such path being:

Code: Select all
`   10007 1 90007 2 90001 3 90901 4 99901 5 99991`

What else would you like to know ?

Leren

<Edit> Corrected error in first path and improved presentation to be more consistent with previous discussions on word ladders.

Leren
Last edited by Leren on Thu Oct 03, 2013 9:29 pm, edited 1 time in total.
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

Leren wrote: This is only required for 1 set of five digit primes, 88259 and 99761

What do you mean?

JPF
JPF
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Location: Paris, France

### Re: A Sobering Problem

JPF wrote:
Leren wrote: This is only required for 1 set of five digit primes, 88259 and 99761

What do you mean?

JPF

What I mean is that my results indicate that if you pick any other pair of 5 digit primes then it is possible to construct a prime number ladder between then with less than 10 numbers
(or 9 steps from the first number in word ladder speak). The other example I gave was 10007 <-> 99991 for which a minimum ladder would have 6 numbers (5 steps from the first number).

Leren
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

How do you go from 10111 to 65563 ?

JPF
JPF
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Posts: 3752
Joined: 06 December 2005
Location: Paris, France

### Re: A Sobering Problem

JPF wrote:How do you go from 10111 to 65563 ?

JPF

A minimum connection path from 10111 to 65563 is as follows:

Code: Select all
`   10111 1 10211 2 12211 3 12251 4 32251 5 32261 6 32561 7 32563 8 62563 9 65563`

I've corrected an error in the longest minimum path which now goes from 88259 <--> 99721. I've edited my original post to reflect this and make the presentation consistent with the style we've been using for word ladders.

Leren
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

My results for connection ladders for 5 digit primes indicate that every prime is connected to every other prime via a minimum sized ladder of at most 10 steps (from the given prime).

The following table shows the number of primes with a given minimum ladder size.

Code: Select all
`Ladder Size        No of Primes     7                  3     8               7650     9                708    10                  2                    -----                     8363`

This shows that every 5 digit prime can be connected to every other 5 digit prime. This is not the case with word ladders - there are lots of words that can't be turned into other words by changing one letter.

So you can obviously produce a ladder that includes all 5 digit primes by changing one digit at each step. What's not obvious is whether you have to repeat a number in the ladder. So the question arises:

Code: Select all
`What is the longest ladder of 5 digit primes that can be made by changing one digit at each step without repeating a number ?`

At the moment I can't answer this question (my code is designed to find shortest paths, not longest paths). I'll give it some thought but in the meantime if anyone can answer this question please fee free to post the answer.

Leren
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

Leren, I have little idea about how your code works, but if you are looping through possible exchange digits it's possible to reduce the candidates to test using the following:

All prime numbers must be either 1 or 2 mod 3
So for example for a 1 mod 3 prime, any 1, 4, or 7 can't be replaced by 0, 3, 6, or 9.
This reduces 9 possible exchanges to 5 or 6
As the options are tested it should therefore be worthwhile to monitor the modulus of the current prime and loop through the candidates in a look-up table to avoid testing non-starters.
Of course for the final digit, the choice is even more restricted.

The only other thought of any merit that I've had is:

Using letters, this family of words provides a very versatile method of extending a ladder
Pat
Pet
Pit
Pot
Put
If any of the words is needed elsewhere it can easily be taken out with no ill effects, and there are also multiple opportunities for managing detours between any two of them.
For the prime ladders it may therefore be worth specifically looking for these and noting the most promising ones.

When checking for a repeated prime a sorted list of those already used may be the way to go. This could include a tag system showing if the prime is necessary or an optional family member.

David
David P Bird
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Joined: 16 September 2008
Location: Middle England

### Re: A Sobering Problem

Here's my first attempt, a ladder sequence of 3624 5 digit primes - no doubt good bed time reading.

Hidden Text: Show
25801 95801 95401 15401 12401 12301 18301 19301 10301 10331 10333 10313 30313 36313 36013 33013 43013 42013 42073
22073 12073 12973 15973 15073 15013 15083 95083 35083 32083 32003 32503 32533 32563 32569 32069 32089 32189 37189
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28901 22901 22961 22963 22973 22573 32573 31573 91573 98573 98473 48473 48073 48079 48049 48649 46649 16649 16619
16319 16519 14519 14549 44549 41549 41149 11149 12149 12109 12809 12899 92899 92893 92693 32693 32993 32933 38933
38903 38803 35803 35603 95603 95203 92203 92503 93503 93553 93053 96053 90053 50053 59053 59093 59393 79393 79397
79337 79357 59357 59377 50377 50777 80777 80747 80347 80387 80687 80681 80611 60611 60661 60161 60761 62761 62861
62801 62401 69401 69403 69473 63473 68473 68477 67477 77477 77977 72977 72997 70997 70991 70921 70321 10321 17321
17021 17027 11027 61027 61057 61051 61091 61001 51001 55001 55051 55061 55661 55691 45691 45697 45667 85667 85627
89627 79627 79687 74687 74587 74507 70507 40507 46507 76507 76597 56597 26597 26497 26437 56437 56431 51431 51421
51481 51581 51551 11551 11251 18251 18253 18257 98257 98227 92227 42227 42221 48221 48281 44281 42281 42283 40283
40883 40813 40013 45013 45053 45953 45959 41959 41659 41609 41809 48809 48889 46889 46819 44819 40819 47819 47869
47569 47563 44563 48563 98563 98543 58543 50543 40543 43543 43943 43973 48973 18973 10973 10993 10093 17093 17293
47293 45293 45233 45433 45413 65413 65213 67213 67219 68219 62219 62213 68213 68813 68819 88819 88919 84919 64919
66919 66949 66947 26947 26347 26317 26357 26387 26987 26981 26681 26881 26801 26501 26701 66701 66601 96601 93601
93607 93407 53407 53401 53101 53201 55201 52201 52301 52361 52321 52391 42391 49391 49891 43891 43801 41801 41201
49201 49211 49811 46811 46817 45817 45317 45377 45677 35677 35671 36671 36871 37871 34871 34877 64877 64879 64871
64171 34171 37171 37181 87181 84181 24181 24121 94121 94621 91621 95621 25621 25601 25609 65609 64609 64679 60679
90679 90379 90373 60373 60383 50383 57383 87383 87583 87523 87623 87629 84629 54629 56629 36629 38629 38639 38039
34039 34939 14939 44939 44839 44809 84809 84869 14869 14867 14767 54767 56767 36767 38767 38567 37567 37537 32537
32531 32561 38561 38261 34261 34361 34381 34781 36781 36721 33721 73721 73121 73421 72421 72221 72229 70229 70223
60223 60923 60913 60917 60017 60317 60217 60257 60457 69457 69857 69877 99877 99577 19577 12577 12547 12647 12347
92347 92317 92357 92353 72353 75353 75553 75583 75983 35983 39983 39383 39323 39343 39043 39023 35023 35027 35227
75227 75527 75521 75511 55511 55411 55711 55717 55787 51787 54787 54287 57287 57487 57467 59467 59447 19447 11447
11467 15467 17467 17167 17117 17137 17737 57737 57731 57751 67751 61751 61651 61681 61381 65381 65581 65981 62981
62987 32987 37987 97987 67987 67957 65957 65951 65851 65881 65831 65731 35731 35771 31771 31751 21751 22751 22721
12721 12781 10781 10181 10141 19141 19441 19421 16421 16921 16927 26927 26627 20627 20327 29327 29347 29147 29137
29131 99131 99191 95191 45191 45197 45127 45137 45737 45767 75767 75787 75781 75721 78721 78121 18121 15121 19121
49121 48121 48821 38821 38861 37861 37061 34061 34961 33961 33931 30931 30911 32911 32971 32371 32377 38377 98377
98387 91387 91381 92381 92581 99581 29581 39581 59581 54581 54181 52181 42181 42101 42901 42961 42461 72461 72431
72031 72931 72911 72211 72251 72253 72953 42953 42923 72923 72823 12823 16823 96823 96821 66821 36821 39821 39841
39241 30241 30941 30949 34949 94949 94349 94049 54049 55049 58049 58043 58013 18013 19013 19073 16073 16063 16763
66763 64763 64793 63793 63799 63199 33199 39199 39799 39769 39569 35569 35069 35059 35759 34759 34729 34129 34123
34183 38183 38153 35153 25153 29153 29173 23173 23473 24473 24443 20443 60443 60449 30449 35449 35149 38149 38189
58189 58199 58193 78193 48193 42193 82193 82183 82153 92153 92143 32143 32443 32413 32423 37423 35423 25423 25453
29453 59453 52453 52457 52757 58757 18757 18457 18427 58427 78427 78467 78497 72497 72493 76493 76403 76003 26003
26083 26783 56783 56773 56473 56453 58453 98453 98443 95443 15443 15473 15173 15773 95773 95873 91873 98873 98893
98993 88993 88493 88463 28463 28163 25163 25183 25189 25889 25849 25841 24841 24821 24421 94421 94321 98321 98621
28621 28661 58661 58631 54631 24631 24611 74611 75611 75011 75013 75083 75883 75853 78853 78823 78893 78593 38593
30593 30293 30893 30853 30851 38851 38351 34351 34301 34303 34403 33403 33703 33503 33563 39563 39163 39133 39233
39239 69239 59239 51239 81239 81299 81199 91199 91129 91139 91159 91459 91499 21499 28499 28439 78439 78479 78179
78139 70139 60139 67139 67129 47129 47123 47143 27143 27103 27109 27179 21179 21149 21139 21839 81839 81869 86869
86969 86959 86939 82939 82039 22039 22739 12739 19739 69739 69439 39439 39419 39499

Leren
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

Here is a list of 6500 five digit primes.

Some issues to resolve to get the full list.

JPF
Attachments
WL6500.txt
JPF
2017 Supporter

Posts: 3752
Joined: 06 December 2005
Location: Paris, France

### Re: A Sobering Problem

It appears that there are 5 five digit primes that are "connected" to only one other five digit prime (by a single digit difference). These are the primes in the first column
of the following table. The primes in the second column are the only ones to which they are "connected".

Code: Select all
`46769   9676997039   3703997919   2791998519   9841999721   99761`

All of the primes in the second column appear in JPF's 6500 prime list and none of the primes in the first column appear in it.

I think this means that, unfortunately, the holy grail of a ladder list of all 8363 five digit primes is impossible - the best you can do would be something like:

46769 96769 .... [ ladder of 8356 primes ... ] .... 99721 99761 - a total ladder of 8360 primes. I have no idea if even this is possible.

Still, it would be nice to find a maximal ladder and demonstrate in some way that it is maximal. Another really cool objective would be to find a maximal cyclic ladder
ie where the last prime in the ladder is 1 digit different from the first prime - so you could start the list from any point along it.

Leren
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

Leren wrote:I think this means that, unfortunately, the holy grail of a ladder list of all 8363 five digit primes is impossible - the best you can do would be something like:
46769 96769 .... [ ladder of 8356 primes ... ] .... 99721 99761 - a total ladder of 8360 primes.
..................

That's a good point.

JPF
JPF
2017 Supporter

Posts: 3752
Joined: 06 December 2005
Location: Paris, France

### Re: A Sobering Problem

I've found that JPF's 6500 prime ladder starts with 87491 and has 17491 at position 6409.

That would make a cyclic ladder of length 6409. There will certainly be other cyclic ladders in that list but it will take a while to check all possibilities to see whether one exceeds a length of 6409.

Leren

<Edit> Now had a chance to check for longer cyclic ladders. 69691 is in position 5 and 79691 is in position 6493 for a longest cyclic ladder length of 6489.

Leren
Last edited by Leren on Sat Oct 05, 2013 9:22 pm, edited 1 time in total.
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

Leren wrote: .... [ a whole bunch of stuff ] ....
In response, JPF and David P Bird wrote: .... [ a lot of other stuff ] ....

I can't believe you guys actually took me up on this. Your next task, should you decide to accept it, is to do the same thing in hexadecimal. Of course, there are m-a-n-y more hexadecimal 5-digit primes than decimal 5-digit primes.

Bill Smythe
Smythe Dakota

Posts: 534
Joined: 11 February 2006

### Re: A Sobering Problem

Here is a 5 digit prime ladder with 7021 numbers starting from the lowest 5 digit prime.

<Edit> Added new file with a 7536 prime number ladder from the same start prime.

Leren
Attachments
Primes 5 Digit Ladder 7536 Numbers.txt
Primes 5 Digit Ladder 7021 Numbers.txt
Leren

Posts: 2902
Joined: 03 June 2012

### Re: A Sobering Problem

Waiting for 8000

edit:
here is a 8154 primes.
There is a cycle 23599 ->33599 with 8139 elements.

JPF
Attachments
WL8154.txt