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Generalizing One-digit products in a row of numbers to base-N:

For which bases N does there exist at least one solution to the following: "The digits from 1 to N can be arranged in a row, such that any two neighbouring digits in this row is the product of two one-digit numbers"?

Examples:

base-10: 728163549
...
base-16: D24E1879A5B6C3F

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  • $\begingroup$ Do you know the answer to this? $\endgroup$
    – Rushabh Mehta
    Sep 23, 2019 at 21:31
  • $\begingroup$ @DonThousand: obviously people can come up with heuristic solutions; I don't see an obvious constructive approach, although there might be one. There seems a pretty obvious generalization of ArnaudMortier's heuristic to "products starting with digit (N-1)" $\endgroup$
    – smci
    Sep 23, 2019 at 21:40
  • $\begingroup$ Did I accidentally use the word 'obvious' three times... doh $\endgroup$
    – smci
    Sep 24, 2019 at 1:15

1 Answer 1

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The problem is not really about products, more about finding an arrangement of digits 1 to N-1 (for base N, since digit N does not exist in base N), that conforms to the criterion that any pair in the arrangement is part of the set of single digit products.

There could be a better way to solve this, but I wrote a small program that creates a table of possible digit combinations and uses that to try to find a valid arrangement.

It's not optimal in terms of performance (essentially a depth-first-search), but finding a pattern for base-N takes N times longer than for base-N-1.

this My program was able to show that bases 3, 4, 5, and 6 do not have such an arrangement (due to digits that do not exist in a product).

Interestingly, for base-2 (binary) it thinks that "1" is a valid combination due to the only non-zero digit being 1, so the row has no neighbouring digits.

Otherwise it shows that bases 7 to 21 have possible arrangements of digits.

base-2:  1 (questionable)
base-3:  -
base-4:  -
base-5:  -
base-6:  -
base-7:  513426
base-8:  5243617
base-9:  46271538
base-10: 728163549
base-11: 739158264a
base-12: 384765a192b
base-13: 35786a294b1c
base-14: 385b27a496c1d
base-15: 375d1b4a698c2e
base-16: 375b6e19c4d2a8f
base-17: 35b8e2ac6f17d49g
base-18: 397af2d6bc5e48g1h
base-19: 37bca59g2d8h1f4e6i
base-20: 35h28be97cf6dag4i1j

Here's a link to a program you can run in your browser.

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  • $\begingroup$ "The problem is not really about products, more about finding an arrangement of digits 1 to N-1" I know that, I already hinted so above. I merely reused the phrasing of @ThomasL's original question for obvious reasons. $\endgroup$
    – smci
    Sep 24, 2019 at 0:48
  • $\begingroup$ The base-2 solution is trivially true, since there's only one non-zero digit. $\endgroup$
    – smci
    Sep 24, 2019 at 0:50
  • $\begingroup$ Sorry, never posted here before, took a while to figure out how to put code in spoiler tags. $\endgroup$
    – Matthew Jensen
    Sep 24, 2019 at 1:07

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