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Online Notes / Linear Algebra / Euclidean n-Space / Euclidean n-Space
Linear Algebra

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In the first two sections of this chapter we looked at vectors in 2-space and 3-space.  You probably noticed that with the exception of the cross product (which is only defined in 3-space) all of the formulas that we had for vectors in 3-space were natural extensions of the 2-space formulas.  In this section we’re going to extend things out to a much more general setting.  We won’t be able to visualize things in a geometric setting as we did in the previous two sections but things will extend out nicely.  In fact, that was why we started in 2-space and 3-space.  We wanted to start out in a setting where we could visualize some of what was going on before we generalized things into a setting where visualization was a very difficult thing to do.

 

So, let’s get things started off with the following definition.

 

Definition 1  Given a positive integer n an ordered n-tuple is a sequence of n real numbers denoted by .  The complete set of all ordered n-tuples is called n-space and is denoted by  

 

In the previous sections we were looking at  (what we were calling 2-space) and  (what we were calling 3-space).  Also the more standard terms for 2-tuples and 3-tuples are ordered pair and ordered triplet and that’s the terms we’ll be using from this point on.

 

Also, as we pointed out in the previous sections an ordered pair, , or an ordered triplet, , can be thought of as either a point or a vector in  or  respectively.  In general an ordered n-tuple, , can also be thought of as a “point” or a vector in .  Again, we can’t really visualize a point or a vector in , but we will think of them as points or vectors in  anyway and try not to worry too much about the fact that we can’t really visualize them.

 

Next, we need to get the standard arithmetic definitions out of the way and all of these are going to be natural extensions of the arithmetic we saw in  and .

 

Definition 2  Suppose  and  are two vectors in .

(a)  We say that u and v are equal if,

                                                 

(b) The sum of u and v is defined to be,

                                              

(c) The negative (or additive inverse) of u is defined to be,

                                                       

(d) The difference of two vectors is defined to be,

                                      

(e) If c is any scalar then the scalar multiple of u is defined to be,

                                                       

(f) The zero vector in  is denoted by 0 and is defined to be,

                                                              

 

The basic properties of arithmetic are still valid in  so let’s also give those so that we can say that we’ve done that.

 

Theorem 1  Suppose ,  and  are vectors in  and c and k are scalars then,

(a)  

(b)  

(c)  

(d)  

(e)  

(f)  

(g)  

(h)  

 

The proof of all of these come directly from the definitions above and so won’t be given here.

 

We now need to extend the dot product we saw in the previous section to  and we’ll be giving it a new name as well.

 

Definition 3  Suppose  and  are two vectors in  then the Euclidean inner product denoted by  is defined to be

                                                    

 

So, we can see that it’s the same notation and is a natural extension to the dot product that we looked at in the previous section, we’re just going to call it something different now.  In fact, this is probably the more correct name for it and we should instead say that we’ve renamed this to the dot product when we were working exclusively in  and .

 

Note that when we add in addition, scalar multiplication and the Euclidean inner product to  we will often call this Euclidean n-space.

 

We also have natural extensions of the properties of the dot product that we saw in the previous section.

 

Theorem 2  Suppose , , and  are vectors in  and let c be a scalar then,

(a)  

(b)  

(c)  

(d)  

(e)  if and only if u=0.