What is the Role of Formal Constructions in Algebraic Equivalence Classes?

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Discussion Overview

The discussion revolves around the concept of "formal" constructions in algebra, particularly in the context of algebraic equivalence classes and the construction of free groups and modules. Participants explore the implications of defining words and sums in algebra without inherent structure, questioning the necessity of precision in these definitions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses discomfort with the term "formal" due to its lack of definition, questioning how to precisely define a "word" in the context of free groups.
  • Another participant argues that a word is simply a combination of letters from a set, emphasizing that the term "formal" indicates the absence of a natural structure for constructing words.
  • A different viewpoint suggests that a "formal" sum of words can be represented as a concatenation of those words, while another participant proposes that it could also be viewed as an element in a group algebra.
  • One participant raises a question about the necessity of treating sequences as functions, citing a specific construction from a textbook that defines formal R-linear combinations in terms of functions with finite support.

Areas of Agreement / Disagreement

Participants express differing views on the definition and implications of "formal" constructions, with no consensus reached on the necessity of precision in these definitions or the best way to conceptualize formal sums and words.

Contextual Notes

Participants highlight the potential for intuition to be misleading when defining formal constructs, suggesting that the lack of a natural operation on strings complicates the discussion.

dmuthuk
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Hi, we often come across certain constructions in algebra that make use of some "formal" sum or "formal" linear combination or "formal" string of elements. Because this term is never defined, I have always been a little uncomfortable when it comes up. For a specific example, consider the construction of the free group on a set X. We begin by defining a "word" in X to be a formal string of elements in X. How do we make this a little more precise? Can we think of a word as an equivalence class of functions into X and concatenation as gluing these functions together? If so, how does that work?
 
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A word is just a word. It is called formal purely because a priori X has no structure that allows us to construct words from the letters. And that is all that is going on. We just think of these things as if they made sense, when there is no innate structure, and then show that it makes sense.
 
If your elements are, say, a, b, c, then a "formal" word is just any combination of those letters. A "formal" sum of "abaca" and "bbac" might be "abacabbac" where the two words are combined in an obvious way.
 
I would have said the formal sum would be abaca+bbac, and an element in the group algebra. Again, we use the word formal simply because addition is not a naturally defined operation on strings.
 
n_bourbaki said:
I would have said the formal sum would be abaca+bbac, and an element in the group algebra. Again, we use the word formal simply because addition is not a naturally defined operation on strings.
That's probably better. I was assuming a specific operation without realizing it.
 
Well, the reason I ask this question is because we sometimes treat sequences as functions from N into a set when it is intuitively obvious what we are talking about. Is this unecessary precision or are there situations where intuition can be misleading? For example, in Dummit and Foote, the free R-module F(A) on a set A is constructed by specifiying the elements of F(A) to be the collection of all set functions f : A --> R with finite support. I guess this makes precise the notion of a "formal" R-linear combination.
 
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