Composite numbers k with no single digit factors whose factors are all substrings of k: Difference between revisions
Composite numbers k with no single digit factors whose factors are all substrings of k (view source)
Revision as of 13:23, 25 January 2022
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Or: 48 out of every 210 positive numbers have no single digit factors.
So, we can generate a few hundred thousand lists of 48 numbers, discard the primes (and 1), then check what's left using substring matching on the factors. (We allow '0' as a 'factor' in our substring test so that we can work with a padded array of factors, avoiding variable length factor lists.)
<lang J> 2{._10 ]\(#~ */"1@((+./@(E. '0 ',])~&>)&:(":&.>)q:))(#~ 1-1&p:)}.,(1+I.0=+/|:4 q:1+i.210)+/~210*i.2e5
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5526173 11610313 13436683 13731373 13737841 13831103 15813251 17692313 19173071 28118827</lang>
Most of the time here is the substring testing, so this could be better optimized.
=={{header|Julia}}==
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