When writing math papers, one sometimes has to refer to some famous theorems, for instance, the Pythagorean theorem.

Shall I write "by Pythagorean theorem", or "by the Pythagorean theorem". When shall I use the article "the"?

  • I am not a mathematician, but did Pythagoras write any theorems other than the one about the square on the hypotenuse? If not, then my inclination would simply be to write re Pythagoras. – WS2 Sep 12 '15 at 23:43
  • 1
    The reference is to a unique theorem, so it should be definite: "by the Pythagorean theorem" or "by Pythagoras' theorem". It's like "the binomial theorem" or "the quadratic formula". – Greg Lee Sep 13 '15 at 7:52

This Google ngram shows what others have done in published books.

Google ngram: by Pythagoras ' theorem,by Pythagorean theorem,by the Pythagorean theorem

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Notice that 'by Pythagorean theorem' has only one occurrence.


"by Pythagorean Theorem"

For example, "I solved this equation using Pythagorean's Theorem," or, I solved expression 6 by using Pythagorean's Theorem."

I hope this helps!😁

  • I'm glad you want to participate, but "Pythagorean's Theorem" is not a valid construction. Maybe "a Pythagorean's Theorem" is theoretically possible, but not for this case. Anyway, people would be left wondering which Pythagorean. – Spencer Mar 31 '18 at 15:10

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