The multiindex notation is an efficient way of denoting several things in multi-dimensional space. For instance, it takes fairly long to denote a partial derivative in the usual way; in the usual notation, a partial derivative is denoted
for some . Now in multiindex notation, the are assembled into a vector , and the term
is then used instead of the partial derivative notation used above. Now, for one partial derivative this may not be a huge advantage (unless one is talking about a general partial derivative), but for instance when one sums all partial derivatives of a polynomial , say, then one obtains expressions as such:
(Note that this is well-defined, as the sum is finite.)
Now compare this to the much longer
as you can see, we saved a lot of time, and that's what's all about. Multiindex notation was invented by Laurent Schwartz.
Other multiindex conventions are the following (we use a convention by Béla Bollobás and denote ):
Multiindex binomial coefficient: Let and be multiindices, then
Multiindex power: Let additionally , then set
Constant multiindex: If , we denote the constant multiindex by the boldface
Multiindex differentiability: We write iff the partial derivatives exist for all with .
Further, the absolute value of a multiindex is defined as
A few sample theorems on multiindices are these (we'll need them often):
Theorem (multiindex binomial formula):
Let be a multiindex, . Then
Note that this formula looks exactly as in the one-dimensional case, with one dimensional variables replaced by multiindex variables. This will be a recurrent phenomenon.
We prove the theorem by induction on . For the case is clear. Now suppose the theorem has been proven where , and let instead . Then has at least one nonzero component; let's say the -th component of is nonzero. Then ( denoting the -th unit vector, i.e. ) is a multiindex of absolute value . By induction,
and hence, multiplying both sides by ,
by the respective rule for the usual -dim. binomial coefficient.
Theorem (multiindex product rule):
Let be a multiindex, be open and . Then
in particular, .
Again, we proceed by induction on . As before, pick such that the -th entry of is nonzero, and define . Then by induction
Note that the proof is essentially the same as in the previous theorem, since by the product rule, differentiation in one direction has the same effect as multiplying the "sum of derivatives" to the existing derivatives.
Note that the dimension of the respective multiindex must always match the dimension of the space we are considering.
Stability properties, TVS of bump functions, convergenceEdit