# isomorphism class representative

• Jan 13th 2013, 07:56 AM
jojo7777777
isomorphism class representative
Why isn't the set {Mmxn|m, nhttp://upload.wikimedia.org/math/7/b...e1a4c7cff0.png ℕ }of isomorphism class representative?
( Mmxn= set of http://upload.wikimedia.org/math/2/5...8e6d1735de.png matrices)
• Jan 13th 2013, 08:36 AM
Plato
Re: isomorphism class representative
Quote:

Originally Posted by jojo7777777
Why isn't the set {Mmxn|m, nhttp://upload.wikimedia.org/math/7/b...e1a4c7cff0.png ℕ }of isomorphism class representative?
( Mmxn= set of http://upload.wikimedia.org/math/2/5...8e6d1735de.png matrices)

Please explain exactly what all of that means.

What is isomorphism class representative?
• Jan 13th 2013, 09:57 AM
jojo7777777
Re: isomorphism class representative
• Jan 13th 2013, 12:00 PM
Deveno
Re: isomorphism class representative
the trouble is, some "elements" (equivalence classes) are listed twice: for example the set of all 2x3 real matrices is isomorphic (as a vector space over R) to all 3x2 matrices (the isomorphism is the transpose map). for a collection of vector spaces to serve as a "class of representatives", we need one and only one from each isomorphism class. that is, the mapping:

NxN-->N

given by (m,n)-->mn is not 1-1 (it is onto, however).

that is (for example): M1x12, M2x6,M3x4,M4x3,M6x2,M12x1 are all isomorphic to R12, so we have "multiple representatives of R12".

the isomorphism classes of FINITE-dimensional vector spaces are characterized by a single facet: their dimension. for an indexed family of vector spaces {Vi: i in I} to serve as a collection of representatives, we need two things:

1) a bijection I <--> N

2) Vi ≅ Vj iff i = j (so that we have a bijection Vi<-->N).

**************************

the entire point of the wikibooks article is this: dimension is the single most important piece of information about a (finite-dimensional) vector space. if i tell you i have a finite-dimensional vector space V over a field F, there's no need to ask "which one" THEY ARE ALL ISOMORPHIC. pick one you like. most people use Fn (n-tuples in F), but you can use polynomials over F of degree n-1 or less if you prefer, or formal linear combinations of any n objects you like.

for example, with n = 3 (and say F = R, for definiteness), it doesn't matter if your basis is:

{red,green,blue}
{1,x,x2}
{(1,0,0),(0,1,0),(0,0,1)}
{1, sin(x), cos(x)}

these all yield isomorphic vector spaces. that's why its important to understand that (a,b,c) does not define a vector. it means av1+bv2+cv3, but first we have to have an agreement what the basis vectors are (so we know what the "coordinates" a,b and c MEAN). a lot of times linear algebra books "gloss over this", by using the standard basis for R3 almost exclusively (it becomes repetitious to say "(a,b,c) in the standard orthonormal basis for R3").

in a sense ai+bj+ck is better, at least some dependence of the basis chosen is being indicated. triples of real numbers ARE NOT VECTORS, until we have decided which basis elements the coordinates "pair to". until then, they are just DATA. choosing a basis is the same thing as choosing "coordinate axes" (1-dimensional linearly independent subspaces of V). but: even a one-dimensional subspace U doesn't "uniquely" determine its basis, we have to decide on WHICH point u in U serves as "the unit distance".

in normal "euclidean space" we sort of cheat: we have geometric ideas of length and angle we use to define "unit lengths" and "perpendicular angles". these are encoded into an "inner product", which in euclidean spaces is the "dot-product". these ideas can be profitably generalized to other types of vector spaces...but vector spaces that are isomorphic as vector spaces may no longer be isomorphic as inner product spaces (an orthogonal basis in one space V, may no longer be orthogonal in that same space, under a different inner product). this is *additional* structure, and not part of the vector space definition.

matrix spaces are often treated as their corresponding "linear" vector space, it is common to "vectorize" matrices by identifying Mmxn(R) with Rmn (this can be thought of as lining up all the columns of a matrix A "head to toe"). surely you can see that both M2x3(R) and M3x2(R) both give us a 1x6 column this way, and so there's no clear choice for "which one" that should serve as the representative for a space of dimension 6.
• Jan 14th 2013, 06:36 PM
LoidaWard
Re: isomorphism class representative
for a collection of vector spaces to serve as a "class of representatives", we need one and only one from each isomorphism class. that is, the mapping:

______________________
Witness Another Success Story for Film Industry in True Blood Season 5 DVD
• Jan 15th 2013, 06:29 AM
jojo7777777
Re: isomorphism class representative
Deveno I'm grateful to you for seriously addressed the question, thank you very much!