Problems & Puzzles: Puzzles

 Puzzle 961. Redo Puzzle 960 Carlos Rivera asks this: Redo Puzzle 960 for K=9 (using only digits 1 to 9). My best and smaller prime solution is this one: Z=1293495697891846825837167457263514231 (37 digits, prime, zero-less-pandigital) Q1. Send your minimal prime solution for K=9. I think that the formula for the minimal quantity of total digits KD needed for a given K value, where K is the quantity of distinct digits, is this one: KD = {K*(K/2), if K=even; K*integer(K/2)+1 if K=odd}...(1) If this is so, the sequence for KD is: KD =  1,2,4,8,11,18,22,32,37,50 ... (2)* for K = 1,2,3,4,5,6,7,8,9,10... Q2. Can you confirm that the formula (1) for the sequence (2) for KD, is the correct formula for our puzzle? ______________ * BTW, (2) is the OEIS sequence A053439. But this sequence is the "Expansion of (1+x+2*x^3)/((1-x)*(1-x^2)^2)" given for "the number of vertices encountered along the shortest walk that encounters every edge at least once on the complete graph with n + 1 vertices. - Peter Kagey, Nov 17 2016"

Contributions came from Claudio Meller, Simon Cavegn, Michael Hürter, Oscar Volpatti, Emmanuel Vantieghem.

BTW, only Simon, Michael and Oscar got the same and minimal prime answer to Q1.

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Claudio wrote on July 14, 2019

Q1. 1231425162718493453647386956792857891

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Simon wrote on July 14, 2019

Q1.
Probably the smallest prime solution:
1231425162718293453647384957658978691

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Michael wrote on July 16, 2019

For Puzzle 961, I found the following solution:

1231425162718293453647384957658978691

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Oscar wrote on July 18, 2019

I computed the sequence of the smallest zero-less prime solutions for 1<=K<=9:

K, L, P:
01, 01,  2
02, 02,  13
03, 04,  1231
04, 08,  12314243
05, 11,  12314253451
06, 19,  1123142345172537457
0722,  1231425162734576356471
0832,  12314234516253645671927493796759
0937,  1231425162718293453647384957658978691

The given sequence (2) for KD is the correct sequence for our puzzle.

In my previous submission, I obtained the given formula (1) as a lower bound for KD:
ZB(K) = max(B1(K), B2(K)),
with:
B1(K) = K(K-1)/2+1,
B2(K) = K*ceil((K-1)/2) = K*floor(K/2).

If K = 2*n, then ZB(K) = B2(K), as:
B1(2*n) = 2*n^2 - n + 1,
B2(2*n) = 2*n^2.

If K = 2*n+1, then ZB(K) = B1(K) = B2(K)+1, as:
B1(2*n+12*n^2 + n + 1,
B2(2*n+1) = 2*n^2 + n.

A nice construction shows that such lower bound can always be achieved.

I computed the smallest zero-less integer solutions for 1<=K<=9.
For K = 2*n, a regular path emerges:

S(2) = 12
S(4) = 12-314234
S(6) = 12-314234-5162536456
S(8) = 12-314234-5162536456-71827384758678

Solution S(2*n+2) is obtained from solution S(2*n) by appending a new block of 4*n+2 digits,
built as follows:
odd positions: digits 2*n+1 and 2*n+2, alternating (with digit 2*n+1 appearing once more);
even positions: digits 1, 2, …, 2*n, then digit 2*n+2.

Within such block, digits 2*n+1 and 2*n+2 appear once contiguous to each other, and once
contiguous to each of the first 2*n digits.
Therefore, the union of the first n blocks is a solution S(2*n) with length 2*n^2.

For 1<=i<=n, digits 2*i-1 and 2*i appear on the boundary of i-th block.
So, if we add a new digit "0" at the beginning of the solution and again at the end of each block,
we obtain a new solution S(2*n+1) with length 2*n^2 + n + 1:

S(1) = 0
S(3) = 0-12-0
S(5) = 0-12-0-314234-0
S(7) = 0-12-0-314234-0-5162536456-0
S(9) = 0-12-0-314234-0-5162536456-0-71827384758678-0

and so on.
However, for K>5, these solutions are not the smallest ones in lexicographical order.

OEIS description for sequence (2) is coherent with our puzzle.

OEIS sequence A053439 is zero-indexed; therefore, the one-indexed sequence KD == a(K-1) is "the
number of vertices encountered along the shortest walk that encounters every edge at least once
on the complete graph with K vertices".
A complete graph has one edge for each pair of vertices (i,j).
A walk can be represented as an ordered list of encountered vertices.
An edge (i,j) is encountered iff its vertices appear in contiguous positions at least once within the list.
Hence, sequence (2) gives the minimal length of an ordered list of vertices, chosen among K vertices,
such that every one of them appears at least once contiguous to each other within the list.

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Emmanuel wrote on July 19, 2019

I think this is the smallest prime: 1231425182716293453847364958756897691

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