A curious math problem I came up with: given a target, what’s the fewest digits an integer must have (in a given base) to contain all integers from 0 to the target, as substrings?
http://wok.oblomov.eu/mathesis/number-substrings/
@mathematics @math@lemmy.ml @math@kbin.social
e.g. for a target of 19 a candidate representative would be 1011213141516171819 in base 10, that has 19 digits. Can it be done in less, or is $\sigma_10(19) = 19$?
Can we find a general rule? Any properties of this function?
@oblomov @mathematics @math@lemmy.ml @math@kbin.social No solution, but the problem is related to de Bruijn sequences (https://en.wikipedia.org/wiki/De_Bruijn_sequence), for which there exists a lot of literature.
@mrdk @mathematics @math@lemmy.ml @math@kbin.social
oh, interesting. It’s definitely related, although we allow different substrings to start at the same place, and this has a huge impact on the lengths (also it’s not cyclic in our case, but that probably makes things worse).@mrdk @mathematics @math@lemmy.ml @math@kbin.social also this might explain why @mau saw some relation to Gray codes in the binary case.
@oblomov well, a Gray code codes all n-bit sequences from 000…0 to 111…1. It’s a bit overkill (we don’t need the sequence with all 0) but probably the overhead is just 1.
