# Homework Help: How to show if a given array of numbers is a vector?

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1. Oct 26, 2015

### shinobi20

1. The problem statement, all variables and given/known data
I'm reading Zee's book Einstein Gravity, I'm in the section where he said that given an array of two numbers p=(ap1, bp2), it is not a vector unless a=b. He just stated it without really showing how it must be like that. I know that a vector should satisfy a transformation p'=R(θ)p with R as the rotation matrix.

In the exercises he also asked to prove that (p2q3, p3q1, p1q2) is not a vector by checking how it transforms under rotation.

2. Relevant equations
p'=R(θ)p with R as the rotation matrix

3. The attempt at a solution
For the first part, p'=Rp yields (ap1cosθ - bp2sinθ, ap1sinθ + bp2cosθ). I'm not sure what this is implying.

2. Oct 26, 2015

### andrewkirk

The question cannot be answered without a great deal more context which, from the sound of it, may need to be the whole containing chapter.

A vector is an element of a vector space, and is defined in relation to that space. To say something is a vector without the context of knowing what vector space we are talking about is like saying a person is a 'member'. A member of what?

Any object can be turned into a vector by constructing a vector space around it, using it as a basis element.

An ordered pair of real numbers is trivially an element of the vector space $\mathbb{R}^2$, but I doubt that's what the author is talking about.

3. Oct 26, 2015

### shinobi20

The author wants to show that it is not a vector if it doesn't preserve the length after rotation. This is tensors actually.