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18 lines
1023 B
Markdown
18 lines
1023 B
Markdown
- [[29]] is [[drishti]] in the #Flancia [[Pattern Language]].
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- My hobby, sometimes: think about whether numbers are prime while laying down or sitting.
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- Take [[209]] -- it is not prime.
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- I find this one quite beautiful, this is how I got there:
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- It is not multiple of two or five trivially.
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- It is not a multiple of three as its digits don't add up to a multiple.
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- It is not a multiple of 7 because 210 is (as 21 is 3 * 7) and it's too near.
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- Consider the hypothesis that it is multiple of 11.
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- 220 is a multiple of 11 because 22 is.
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- 220 - 11 is 209.
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- So 209 is a multiple of 11. What is the other factor?
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- Consider the hypothesis that it is a multiple of 19.
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- 19 * 10 is 190.
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- 190 + 19 is 209 precisely, so it is a multiple.
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- Therefore 209 is 11 * 19.
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- [[1547]] is not prime; it is 7 * 13 * 17.
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- I got to this one by whistling a 1547 tone, seeing it in [[spectroid]], then checking https://anagora.org/prime/1547 :)
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