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Equation 45 · Part 4 · The Bend an Elevator Cannot Fake

subscript

ϕi=ϕj\phi_i = \phi_j
subscript

What this part means

The lower label selects a particular version, component, or indexed member of the quantity. For example, x₀ and xₜ can be values at different positions.

Its job in the formula

A subscript distinguishes a version, component, step, or member of a quantity. It does not automatically mean multiplication.

The passage around this formula

What r is actually sensitive to follows from the same argument in reverse. The null space of B has dimension equal to the number of connected components of the network graph; for a connected network it is exactly one-dimensional, spanned by the all-ones vector, because Bϕ\phi = 0 forces ϕi\phi_i = ϕj\phi_j across every edge and connectivity propagates that equality to every node. By rank–nullity, a connected tree — with exactly |V|-1 edges for |V| nodes — has an incidence matrix of full row rank, meaning that for any data whatsoever a ϕ\phi exists reproducing it exactly. A tree, in other words, cannot fail to close, on any data, ever; it carries zero redundant edges and therefore zero capacity to…

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Learn the underlying idea

A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

Open the illustrated subscripts: which member of a family? guide →

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