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Several students were asked the following problem:

Prove that all odd integers are prime.

Well, the first student to try to do this was a math student. Hey
says "hmmm... Well, 1 is prime, 3 is prime, 5 is prime, and by
induction, we have that all the odd integers are prime."

Of course, there are some jeers from some of his friends. The
physics student then said, "I'm not sure of the validity of your proof,
but I think I'll try to prove it by experiment." He continues, "Well, 1
is prime, 3 is prime, 5 is prime, 7 is prime, 9 is ... uh, 9 is an
experimental error, 11 is prime, 13 is prime... Well, it seems that
you're right."

The third student to try it was the engineering student, who
responded, "Well, actually, I'm not sure of your answer either. Let's
see... 1 is prime, 3 is prime, 5 is prime, 7 is prime, 9 is ..., 9 is
..., well if you approximate, 9 is prime, 11 is prime, 13 is prime...
Well, it does seem right."