Paper VIII visual guide · Visual 21 of 32 · THREE EDGES, TWENTY-SEVEN ASSIGNMENTS

An exact budget is not a greedy choice

Traditional p. 29 / Theorem 8.7, Remark 8.8.

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Each edge has three allowed \((\text{integer weight},\text{cost})\) options:

\[ \begin{aligned} 1 &: (0,7),(2,3),(4,1),\\ 2 &: (0,6),(1,4),(3,1),\\ 3 &: (0,9),(2,5),(5,1). \end{aligned} \]

There are exactly \(3^3=27\) full assignments. The table stores the minimum cost at each exact weight; columns zero through eight are displayed.

Dynamic-programming costs by exact integer weight
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Columns are exact integer weights and rows are successive numbers of processed edges. Each cell gives the minimum declared cost at that exact weight; infinity marks an unreachable state. The budget optimum is the minimum across the allowed columns.

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Drawing grid 1 · rows top to bottom, columns left to right
Row / column012345678
edges 00\(\infty\)\(\infty\)\(\infty\)\(\infty\)\(\infty\)\(\infty\)\(\infty\)\(\infty\)
edges 17\(\infty\)3\(\infty\)1\(\infty\)\(\infty\)\(\infty\)\(\infty\)
edges 213119774\(\infty\)2\(\infty\)
edges 32220181614121298

Columns are exact weights; the budget optimum is the minimum across allowed columns.

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At budget four, minimize across columns zero through four. The optimum costs 14: choose weights \((2,0,2)\) with costs \((3,6,5)\). The zero-weight choice on edge two is its first, more expensive option. A locally cheap choice can consume too much of the shared budget.

The checker compares the recurrence with complete enumeration for every budget from zero through twelve. Infeasible states remain \(+\infty\); they are not zero-cost states.

Quantization must be conservative

Suppose real errors are \(3/50\) and \(2/25\), and one integer unit represents \(1/10\). Rounding down gives two zero weights, even though the true sum is \(7/50>1/10\). Rounding up gives a certified upper envelope instead. The budget method is exact for its integers; it cannot repair unsafe integers supplied by a preprocessing step.

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The table optimizes the declared option set exactly. Accuracy depends separately on the validity and conservativeness of the option weights.

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