Paper X visual guide · Visual 18 of 30 · EXACT JOINT PROJECTORS

Pairwise occupancy can still fail jointly

Main text: Theorem 10.1 and the finite occupancy models.

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Let a six-dimensional rational carrier have the joint eigenvalue labels shown below for three commuting diagonal operators \(T_1,T_2,T_3\). Each coordinate is zero or one. Selecting a zero uses \(I-T_i\); selecting a one uses \(T_i\).

Pairwise occupied eigenspaces with empty joint occupancy
Original drawing. Read the description below; expand the labels and data for selectable text.

Reading the drawing

The occupancy table has six states as columns. Its rows compare successive zero-eigenvalue restrictions with the missing target 011. Each pair of the target requirements has an occupied state, while all three together have none. The example therefore keeps joint occupancy explicit.

Drawing labels and data 17 labels

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  • \(000\)
  • \(001\)
  • \(010\)
  • \(101\)
  • \(110\)
  • \(111\)
  • all states
  • \(6\)
  • \(T_1=0\)
  • \(3\)
  • \(T_1=T_2=0\)
  • \(2\)
  • \(T_1=T_2=T_3=0\)
  • \(1\)
  • target \(011\)
  • \(0\)

Each pair of requirements from \(011\) is occupied. Their triple intersection is empty.

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The full conjunction is a different test

For target \((0,0,0)\), successive conditions reduce the ranks from \(6\) to \(3\), then \(2\), then \(1\). The joint projector is nonzero, so that label is occupied in this carrier.

For target \((0,1,1)\), every pair of conditions is occupied: \((0,1,0)\) witnesses the first two, \((0,0,1)\) the first and third, and \((1,1,1)\) the second and third. But the triple itself is absent. Multiplying all three selectors gives the zero matrix.

\[ E_{\boldsymbol a}=\prod_{i=1}^{3} \begin{cases}I-T_i,&a_i=0,\\T_i,&a_i=1.\end{cases} \qquad {\operatorname{Tr}}(E_{\boldsymbol a})={\operatorname{rank}}(E_{\boldsymbol a}). \]

The checks compute all eight joint projectors, their pairwise orthogonality, and their sum. The sum is the identity even though two labels are absent: those two projectors are simply zero.

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A formula for a projector does not force its image to be nonzero. Occupancy must be tested on the complete set of required conditions.

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