Talk:Untouchable numbers: Difference between revisions
→Nice recursive solution
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:I was thinking along (vaguely) similar lines, seeing it spelt out like that clarified a couple of points:
:It is guaranteed to iterate through all composite numbers (as long as we carry on until Σ too big), and we have no interest in (large) primes.
:Obviously it would be very silly to use trial division when we already have all the prime factors.
:However, I am sure there is an easy/clever way to calculate Σ, from those primes [and/or prev Σ?], but I am ashamed to say I just can't see it.
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