Cauchy-Schwartz Theorem
February 28th, 2008 by WaltWhile I was driving in my car today, I thought of a proof of the Cauchy-Schwartz theorem. I’m sure that it is completely unoriginal, but it has the advantages of both being longer and requiring more background than the usual proof (which you can find on the Wikipedia page).
Let (,) be an inner product. From the definition, we know that for two vectors x, y and two scalars a, b that
(ax+by, ax+by) = a2 (x,x) + 2ab (x,y) + b2 (y,y) ≥ 0
This is a positive-definite quadratic form in a, b, which means that its associated matrix has positive determinant:
(x,x) (y,y) - (x,y) (x,y) ≥ 0,
which is the result.
The real advantage of the proof, I suppose, is that if you already have the linear algebra background there’s no trick involved. It also means that using the same determinant argument there are analogues of the inequality that involve n vectors instead of two.
February 28th, 2008 at 11:01 pm
It is nice. I read somewhere that it is exactly the way Schwarz proved it. Actually you don’t need the full-blown linear algebra; there is the concept of discriminant for quadratic equations which is the same as your 2×2 determinant.
March 7th, 2008 at 11:34 am
See Lax’s Linear Algebra proof of the same.
March 19th, 2008 at 9:52 pm
I wondered if this was the original proof: it has a very classical flavor.
April 9th, 2008 at 5:37 am
Nice proof, and It can be modified in order to work in the complex case.