Problems & Puzzles: Puzzles

 Puzzle 1031. Rigidly-deletable primes again Arkadiusz Wesolowski sent the following nice puzzle   For an positive integers k and n with 1 < k < n and k|n, let P(n, k) be the smallest n-digit prime with the property that deleting some k digits, not necessarily contiguous, one at a time in unique order gives a prime at each step, until the empty string is reached. Note: leading zeros are allowed.   Examples for k=2: a) P(4, 2) = 1009, because 1009 is primes and only 1XX9 is prime. b) (P(4, 2))=2657 because 2657 is prime and the set {57, 67, 65, 27, 25, 26} (step 1) contains only one prime.   Rigidly-deletable primes for k=1 are in A188809. Q. Send your largest examples for 1 < k < n and k|n . ____ This puzzle was edited on Sunday 7 of March, 2021.

During the week 5-12 March, 2021, contributions came from Emmanule Vantieghem,

***

Emmanuel wrote.

These are the longest chains I could find:

P(12, 2) = 100140952927.
100140952927   >>   10014*952*27 = 1001495227   >>   1**1495227 = 11495227   >>   11*9*227 = 119227   >>   **9227 = 9227   >>   9**7 = 97.
(I'm quite sure there is no smaller 12-digit number that gives a chain of that length).

P(12, 3) <= 111444884899
111444884899   >>   ***444884899 = 444884899   >>  ***884899 = 884899   >>  **4*99 = 499.

(I think It is possible that there is a smaller 12-digit number that gives a chain of that length).

...

I changed my strategy so that now my best result is :

P(28, 2) = 8983300668045233737988665427.
The corresponding chain is :
8983300668045233737988665427   >>   89833006684523377988665427   >>   893300668453377988665427   >>   8330066845337798866427   >>   83300668533779886647   >
>   833006653377988667   >>   8006653377988667   >>   80066533779887   >>   800533779887   >>   8533779887   >>   85779887   >>   779887   >>   9887   >>   97

I restricted myself to numbers with an even number of digits, i. e. : I did not allow initial zeros.  So, it is possible that there exist 'smaller' solutions with longer chains.

If you want a table  { P(2n,2) |  n = 1  to  13 }, let me know.

...

n       P(2n, 2)
2       1009
3       100801
4       10131557
5       1001495227
6       100140952927
7       10013551285567
8       1009135512785567
9       101499476982207001
10      10331914669692209647
11      1137899646227100132401
12      166397047330565527043201
13      34344792550711992070815007
14      8983300668045233737988665427

....

{1009, 19}
{100801, 1801, 11}
{10131557, 113557, 3557, 37}
{1001495227, 11495227, 119227, 9227, 97}
{100140952927, 1001495227, 11495227, 119227, 9227, 97}
{10013551285567, 113551285567, 1135512557, 11312557, 113557, 3557, 37}
{1009135512785567, 10013551285567, 113551285567, 1135512557, 11312557, 113557, 3557, 37}
{101499476982207001, 1149947692207001, 11499479227001, 114994722001, 1199422001, 11994001, 114001, 4001, 41}
{10331914669692209647, 103311466969220647, 1331146699220647, 33114669922067, 331146622067, 1146622067, 11662267, 662267, 2267, 67}
{1137899646227100132401, 11378996422100132401, 113996422100132401, 1199642210013241, 11994221001341, 119942210041, 1199422001, 11994001, 114001, 4001, 41}
{166397047330565527043201, 1669707330565527043201, 16697733056557043201, 669773306557043201, 6677330655043201, 66770655043201, 667765504201, 6677655001, 66776551, 776551, 6551, 61}
{34344792550711992070815007, 343449255071199207081007, 3344925507119920708007, 33449207119920708007, 334492011992008007, 3344920119928007, 33449011992007, 334490119927, 3344901199, 33449011, 449011, 9011, 11}
{8983300668045233737988665427, 89833006684523377988665427, 893300668453377988665427, 8330066845337798866427, 83300668533779886647, 833006653377988667, 8006653377988667, 80066533779887, 800533779887, 8533779887, 85779887, 779887, 9887, 97}

Later he added on March 19, 2021:
I found the exact value of  P(12, 3) :

P(12, 3) = 111099000109
with sequence :
111099000109   >>   111*990001** = 111990001  >>   ***990001 = 990001   >>   99***1 = 991

(We have also :
P(9,3) = 100220009   with sequence
100220009, 220009, 229
and  P(6, 3) = 100669  with sequence
106669, 109)

Then on March 26, he added:

P(15, 3) = 102288899440009  with sequence :
102288899440009  >>  1*2288899**0009 = 122888990009  >>  122***990009 = 122990009  >>  *22**0009 = 220009  >>  22***9 = 229

and

P(18, 3) = 866655665488832627  with sequence
866655665488832627  >>  866655665*888**627 = 866655665888627  >>  *6665566*888*27 = 666556688827  >>  66655**888*7 = 666558887  >>  66655***7 = 666557  >>  ***557 = 557.

Moreover, P(21, 3)  does not exist.
Neither does  P(30, 2).

***

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