 # Glas :

## Re: [glas] norms and inner products for vectors of matrices

From: Karl Meerbergen (Karl.Meerbergen_at_[hidden])
Date: 2006-01-13 11:12:40

Peter Gottschling wrote:

>On 13.01.2006, at 05:02, Karl Meerbergen wrote:
>
>
>
>>Peter Gottschling wrote:
>>
>>
>>
>>>Concerning the norms, I would like to limit the result type to real
>>>values as I have seen it in all mathematical definitions. Does
>>>somebody sees a reason to vectors or matrices as results?
>>>
>>>The definitions of vector<vector> norms are straight forward:
>>>- norm_1 := sum (norm_1(x[i])
>>>- norm_2 := sqrt(sum(norm_2(x[i])^2)) // there might be
>>>more
>>>efficient ways to compute this
>>>- norm_inf := max(norm_inf(x[i])
>>>
>>>Best,
>>>Peter
>>>
>>>
>>>
>>>
>>>
>>>
>>I am afraid that we cannot define norm_1, etc in this way. (I too made
>>this mistake in a previous mail) But we can define functions
>>generalized_norm_1 := sum (generalized_norm_1(x[i])
>>generalized_norm_2 :=sqrt( sum (generalized_norm_2(x[i])^2 )
>>generalized_norm_inf := max (generalized_norm_inf(x[i])
>>
>>norm_1, norm_2, norm_inf are well defined in linear algebra and these
>>definitions should be respected:
>>norm_1 := sum( abs(x[i]) )
>>etc.
>>These should only be used for vector and matrix objects with 'scalar'
>>value_type's.
>>
>>
>>Karl
>>
>>
>>
>Karl, I agree that it is not a good idea to call always the recursive
>definition.
>
>(Even if I think it is possible by defining norm_1 of scalars as abs
>etc. Anyway, it's ugly.)
>
>The way I would like to define it is to have one function and dispatch
>first between scalar value types and nested value types. For instance
>
>template <typename Vector>
>double norm_1(const Vector& v, type::scalar)
>{
> return sum(abs(x[i])); // well this is only pseudo-code
>}
>
>template <typename Vector>
>double norm_1(const Vector& v, type::nested)
>{
> return sum(norm_1(x[i]));
>}
>
>template <typename Vector>
>double norm_1(const Vector& v)
>{
> nesting_traits<typename Vector::value_type>::type nesting;
> return norm_1(v, nesting);
>}
>

Agreed. Actually, the current implementation defines norm_1=abs for a
scalar. The nesting_traits is not required.

>
>With this code we have the classical defintion if the value_type is
>scalar and the recursive definition is the value_type itself is a
>vector. Note that this also handles higher levels of nesting than 2.
>
Agreed. The current implementation uses a similar mechanism

>
>What remains is the question how we define norm_1(vector<matrix<T> >) ?
> I'm not convinced that my definition is perfect for this. OTOH, I
>wonder if there is a reasonable norm definition for vector<matrix> ????
>

sum(norm_1(v[i]) is a norm, there is no doubt about that. But there is
no correspondance with the algebraic matrix 1-norm.

>
>May be we should discuss first what we consider the norm of a matrix.
>Matrices are typically used as linear operators and operator norms can
>be defined in terms of vector norms norm_xx(op)=sup( norm_xx( op(v) ) /
>norm_xx (v) ). I vote for defining matrix norms as operator norms.
>

Agreed. That is the way matrix norms are defined. We should stay as
close as possible to the operator norms.

>OTOH, matrices model the concept of VectorSpace and it is
>mathematically perfectly correct to apply the vector norm directly. To
>do this I suggest to use views for it, e.g.
>
>vector_view<matrix<...> > vv(m);
>alpha= norm_xx(vv);
>
>One complication I see is that nested matrices has to be considered as
>nested vectors, in other words the view must be applied recursively.
>

I do not see a problem with that. The same holds for all functions, e.g.
trans, herm, conj, etc

>
>Coming back to the question what is norm_xx( vector<matrix> ) it would
>be a mixture of vector and operator norms, which doesn't look
>reasonable to me. Probably we see it clearer if we know what usage
>vector<matrix> have (if there is any).
>

It depends what the user wants to do with the nested matrix. If there is
an operator behind, the user can create a norm for that. Unfortunately,
for such interpretations, GLAS cannot provides the functions, but GLAS
can support the creation of such functions in an easy way.

Conclusion;
* we keep the current implementation norm_1=sum(norm_1(v[i]) ?
* If the user wants a more appropriate norm for his application, he will
have to write it. As Toon mentioned, we cannot support all possible
interpretations of nested collections. (But we can provide tools to
develop appropriate functions). Recall that the collection attributes
are very powerful for that.

Karl

>
>Best,
>Peter
>------------
>Peter Gottschling
>Research Associate
>Open Systems Laboratory
>Indiana University
>301i Lindley Hall
>Bloomington, IN 47405
>Tel.: +1 812 855-8898 Fax: +1 812 856 0853

>
>_______________________________________________
>glas mailing list
>glas_at_[hidden]

>
>
>

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