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Bachman on chains 
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#1
May1111, 12:37 PM

P: 1,402

Bachman: A Geometric Approach to Differential Forms, p. 65:
(2) What are n and m: integers, rational numbers, real numbers? (3) Is it also assumed that m(S + T) = mS + mT, and does 1S = S? The numbers 1 and 1 distribute over cells in his definition of the boundary of a cell, and I think he assumes that the latter relation holds too. This would imply that kchains make a vector space or a module, if m and n represent elements of a field or ring respectively. (NOTE: Bachman writes nchain and ncells; I changed this to k, as I'm guessing the n in the name has no connection to the n in his second relation.) 


#2
May1211, 12:58 PM

P: 1,402

This source also uses the expression "formal linear combination" in defining a chain (also without definition), although here the elements combined are maps from "psimplices" that are subsets of R^{p} to a manifold M, rather than the images of unit cubes in R^{p} under a map to M. He also says, "Boundaries are a subset (in fact, a subvector space) of the cycles" (p. 64), and the cycles are a subset of the chains. So, reading between the lines, perhaps Bachman's chains and this guy (Pemantle)'s chains are vectors, in which case the coefficients of Bachman's cells would belong to a field, such as the rationals or the reals.



#3
May1211, 05:23 PM

Sci Advisor
P: 1,722

To be precise the formal module is all formal linear combinations of the simplices modulo the equivalence relations, m(S + T) = mS + mT and so forth. 


#4
May1311, 06:09 AM

P: 1,402

Bachman on chains
Thanks, lavinia. To summarise, the structure being defined by Bachman is a Zmodule, meaning a module over the integers. Its elements are chains of cells with integer coefficients. "Formal" is (operationally) superfluous in formal linear combination, like the "linear" in "linear vector space"? But perhaps, although it doesn't denote any difference, it carries a connotation of being "merely symbolic". Pemantle defines his chains slightly differently with maps called simplices in place of Bachman's sets called cells. He probably allows the coefficients to be at least rationals, possibly reals, since he describes chains as vectors.



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