Completing the Square for Quadratic Forms

[Entertainment]

Homework Statement

Complete the square to determine the signature of the following quadratic forms:

a) h(x,y) = xy
b) h(x,y,z) = yz
c) h(x,y,z) = x^2 + y^2 + z^2 + 2xy + 2xz + yz

Homework Equations

The Attempt at a Solution

I suspect there's a really simple trick to this, but I can't seem to figure it out at the moment.

For c), we get h(x,y,z) = x^2 + 2xy + 2xz + y^2 + yz + z^2 = x^2 + 2x(y+z) + y^2 + yz + z^2 = (x+y+z)^2 - (y+z)^2 + y^2 + yz + z^2 = (x+y+z)^2 - yz, but I suspect that would require a solution of b) to get further.

 

[fzero]

Can you write xy as a sum of quadratic terms? Hint: You might have to square the sum of two things.

 

[Entertainment]

Can you write xy as a sum of quadratic terms? Hint: You might have to square the sum of two things.

After messing around a bit, I found that

a) xy = (x+\dfrac{y}{4})^2 - (x-\dfrac{y}{4})^2
Signature: (1,-1)

b) Similar to a): yz = (y+\dfrac{z}{4})^2 - (y-\dfrac{z}{4})^2
Signature: (1,-1,0)

c) Continuing from before, and using b); (x+y+z)^2 - yz = (x+y+z)^2 + (y-\frac{z}{4})^2 - (y+\frac{z}{4})^2
Signature: (1,1,-1)

Am I overlooking something?

 

[fzero]

The signature of a quadratic form is usually defined as the number of positive eigenvalues minus the number of negative eigenvalues of the corresponding matrix. You can compute this from your results.