John Conway proves that 91 is the smallest number which looks prime but isn't

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Some superstitious nerdy teenagers convinced small child me that 91 was a universal anomaly bc it’s 13 (bad luck) * 7 (good luck), so my weird brain never saw 91 as prime , & I can’t be the only person this happened to
No it's 51
- Naw, 5 + 1 = 6, 6 is divisible by 3, so 51 is divisible by 3.
  
  I shared this at work recently and nobody cared. 3 6 and 9 are all kinda special.
  
  17
  
  Sure, and 91 is divisible by 7 if you know your 7 times tables.
- Only if you weren't taught (or were, but too long ago lol) the divisible by 3 rule - if the sum of the digits is divisible by 3, so is the whole number. 5+1=6 therefore 51 is divisible by 3! Bonus division rule - if it was also an even number and therefore divisible by 2, you could deduce that the whole number was divisible by 6.
  
  Actually 51/3! Is 8.5
  
  Actually, 51 is not divisible by 3!
  
  It is divisible by 3
- Surely it's 1? Looks prime, and some people will even argue that it is!
  
  Its only factors are 1 and itself after all
- Agreed. 51 looks prime AF.
I always felt that 21 should be, but nooo. Stupid 7
- 21 should be
  
  2 + 1 = 3, therefore 21 is divisible by 3 and is not prime. QED :)
  
  9+1 = 10, therefore 91 is divisible by 10 and is not a prime ... QED, I guess
It's funny, but actually a good proof

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