A Mathematician, a Physicist and an Engineer entered a mathematics contest, the first task of which was to prove that all odd numbers are prime. That's easy said the Mathematician; "1's a prime, 3's a prime, 5's a prime, 7's a prime. Therefore by mathematical induction, all odd numbers are prime". Then it was the Physicist's turn: "1's a prime, 3's a prime, 5's a prime, 7's a prime, 11's a prime, 13's a prime. So, allowing for experimental error, all odd numbers are prime. The engineer provided the most straightforward proof: "1's a prime, 3's a prime, 5's a prime, 7's a prime, 9's a prime, 11's a prime, 13's a prime . . . ".

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