## Tuesday, November 17, 2015

### [U_Complex20151117] Lagrange Identity for Complex Numbers

Question

Introduction
I was revising my complex analysis just for fun (I had graduated almost 3 decades ago) and I came across this problem which is question 5 from page 9 of the book Complex Analysis by Ahlfors.  This book is pretty hard core for a first course in complex analysis, but this is not surprising since Lars Ahlfors was no less than a Field’s medallist.
Intuitively, I knew that this identity looks like the vector identity
|a · b|2 = |a|2 |b|2 – |a ´ b|2
and if one strips away the |a|2 |b|2, it boils down to the Pythagorean Trigonometric Identity
cos2 q = 1 – sin2 q
I thought if I could just define the appropriate dot and cross products (actually this can be done), I could solve it easily.  However, this itself requires proof.  I was begging the question.  In fact, it is precisely because of this Lagrange Identity (and the related Cauchy-Schwarz Inequality) that allows  cos q  and   sin q  to be meaningfully defined.
OK, it looks like I have to do it the hard way.  The tricky part in the manipulation of those sums in sigma notation is to ensure, at each step, that I did not introduce any spurious terms, nor miss out any terms.  To simplify the notation, in what follows I shall assume that  i  and  j  are indices in the range of whole numbers  [n] = {1, ... , n}.

Solution

Remarks

H04. Look for pattern(s)
H05. Work backwards
H09. Restate the problem in another way
H13* Use Equation / write a Mathematical Sentence

Suitable Levels
University / College level Complex Analysis
* other syllabuses that involve Complex Analysis

* any independent learner game for a challenge