Paper I visual guide · Visual 9 of 22 · REACHABILITY / NECESSARY IS NOT SUFFICIENT

Two visible routes can cancel exactly

Main text: Corollary 8.4 and Proposition 9.3

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Let a single operator \(D\) on \({\mathbb Q}^4\) act by

\[ De_1=e_2+e_3,\qquad De_2=e_4,\qquad De_3=-e_4,\qquad De_4=0. \]

Every vertex of its transfer graph for the coordinate packets is reachable from vertex \(1\). Nevertheless,

\[ D^2e_1=e_4-e_4=0,\qquad D^2=0. \]
Two weighted paths cancel in one transfer block
Original drawing. Read the description below; expand the labels and data for selectable text.

Reading the drawing

A weighted diamond gives two routes from the first vertex to the fourth. Their signed contributions cancel. The adjacent Krylov matrix records the resulting orbit and its rank.

Drawing labels and data 23 labels

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  • \(e_1\)
  • \(e_2\)
  • \(e_3\)
  • \(e_4\)
  • \(+1\)
  • \(+1\)
  • \(+1\)
  • \(-1\)
\([e_1,De_1,D^2e_1]\)
RowColumn 1Column 2Column 3
Row 1\(1\)\(0\)\(0\)
Row 2\(0\)\(1\)\(0\)
Row 3\(0\)\(1\)\(0\)
Row 4\(0\)\(0\)\(0\)
  • rank \(=2\)
  • \((D^2)_{41}=1-1=0\)

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Evidence note from the source edition

The diamond is the actual weighted action of \(D\). The two paths of length two exist, but their contributions to the same transfer block sum to zero.

What the seed generates

With the fixed algebra \({\mathbb Q}[D]\) and seed \(e_1\),

\[ {\operatorname{Hull}}_{{\mathbb Q}[D]}(e_1)={\operatorname{span}}\{e_1,e_2+e_3\}. \]

It has dimension two, not four. Both the independent difference \(e_2-e_3\) and the last coordinate \(e_4\) are missing.

Why projecting in the middle changes the experiment

If \(P_2\) becomes an admissible primitive, the word \(DP_2D\) satisfies

\[ DP_2De_1=DP_2(e_2+e_3)=De_2=e_4. \]

But \(P_2\) was not an allowed generator of the original \({\mathbb Q}[D]\) grammar. Using it to argue that the original seed already reached \(e_4\) would silently enlarge the algebra.

Read this as

Unreachable nonzero packets obstruct generation. Reachability by itself does not prove generation: coefficients can cancel, blocks can have small images, and intermediate projectors may not be admissible.

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