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595: 現代数学の系譜 雑談 ◆yH25M02vWFhP  [] 2020/10/25(日) 09:23:50.10 ID:eIdDsFH8

>>594
つづき

In this context, we remark that it is also this state of affairs that gave rise to the term
“inter-universal”: That is to say, the notion of a “universe”, as well as the use of
multiple universes within the discussion of a single set-up in arithmetic geometry, already
occurs in the mathematics of the 1960’s, i.e., in the mathematics of Galois categories
and ´etale topoi associated to schemes. On the other hand, in this mathematics of the
Grothendieck school, typically one only considers relationships between universes − i.e.,
between labelling apparatuses for sets − that are induced by morphisms of schemes, i.e.,
in essence by ring homomorphisms. The most typical example of this sort of situation
is the functor between Galois categories of ´etale coverings induced by a morphism of
connected schemes. By contrast, the links that occur in inter-universal Teichm¨uller
theory are constructed by partially dismantling the ring structures of the rings in their
domains and codomains [cf. the discussion of §2.7, (vii)], hence necessarily result in
much more complicated relationships between the universes − i.e., between the labelling apparatuses for sets − that are adopted in the Galois categories that occur in the domains and codomains of these links, i.e., relationships that do not respect the various labelling apparatuses for sets that arise
from correspondences between the Galois groups that appear and the respective
ring/scheme theories that occur in the domains and codomains of the links.

つづく

http://rio2016.5ch.net/test/read.cgi/math/1592654877/595

596: 現代数学の系譜 雑談 ◆yH25M02vWFhP  [] 2020/10/25(日) 09:24:07.43 ID:eIdDsFH8

>>595
つづき

That is to say, it is precisely this sort of situation that is referred to by the term
“inter-universal”. Put another way,
a change of universe may be thought of [cf. the discussion of §2.7, (i)] as
a sort of abstract/combinatorial/arithmetic version of the classical notion
of a “change of coordinates”.
In this context, it is perhaps of interest to observe that, from a purely classical point of
view, the notion of a [physical] “universe” was typically visualized as a copy of Euclidean
three-space. Thus, from this classical point of view,
a “change of universe” literally corresponds to a “classical change of the coordinate system − i.e., the labelling apparatus − applied to label points in
Euclidean three-space”!
Indeed, from an even more elementary point of view, perhaps the simplest example of the
essential phenomenon under consideration here is the following purely combinatorial
phenomenon: Consider the string of symbols
010
− i.e., where “0” and “1” are to be understood as formal symbols. Then, from the
point of view of the length two substring 01 on the left, the digit “1” of this substring
may be specified by means of its “coordinate relative to this substring”, namely, as the
symbol to the far right of the substring 01. In a similar vein, from the point of view of
the length two substring 10 on the right, the digit “1” of this substring may be specified
by means of its “coordinate relative to this substring”, namely, as the symbol to the far
left of the substring 10. On the other hand,
neither of these specifications via “substring-based coordinate systems”
is meaningful to the opposite length two substring; that is to say, only the
solitary abstract symbol “1” is simultaneously meaningful, as a device
for specifying the digit of interest, relative to both of the “substring-based coordinate systems”.

つづく

http://rio2016.5ch.net/test/read.cgi/math/1592654877/596

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