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 13:21, 25 January 2022
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5526173 11610313 13436683 13731373 13737841 13831103 15813251 17692313 19173071 28118827
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=={{header|J}}==
<lang J> */2 3 5 7
210
#1+I.0=+/|:4 q:1+i.210
48</lang>
Or: 48 out of every 210 positive numbers have no single digit factors.
So, we can generate a few hundred thousand 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
15317 59177 83731 119911 183347 192413 1819231 2111317 2237411 3129361
5526173 11610313 13436683 13731373 13737841 13831103 15813251 17692313 19173071 28118827</lang>
=={{header|Julia}}==
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