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- [[29]] is [[drishti]] in the #Flancia [[Pattern Language]].
- My hobby: think about whether numbers are prime.
- [[209]] is not prime; it is 19 * 11.
- I find this one quite beautiful as there are two simple "proofs" to get there:
- My hobby, sometimes: think about whether numbers are prime while laying down or sitting.
- Take [[209]] -- it is not prime.
- I find this one quite beautiful, this is how I got there:
- It is not multiple of two or five trivially.
- It is not a multiple of three as its digits don't add up to a multiple.
- It is not a multiple of 7 as 210 is (as 21 is 3 * 7); and it's too near to it.
- It is not a multiple of 7 because 210 is (as 21 is 3 * 7) and it's too near.
- Consider the hypothesis that it is multiple of 11.
- 220 is a multiple of 11.
- 220 - 11 is 209.
- So 209 is a multiple of 11. What is the other factor?
- Consider the hypothesis that it is a multiple of 19.
- 19 * 10 is 190.
- 190 + 19 is 209 precisely, so it is a multiple.
- Therefore 209 is 11 * 19.
- [[1547]] is not prime; it is 13 * 7 * 17.
- I don't remember how I got here but it was fun.