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Past exam of the mathematics course of the University of Cambridge / 2022 / ii / Paper 2 / 31J / c / Solution

Let

Z = (X, Y)

be an independent test example and write

Z_{i} = (X_{i}, Y_{i})

. Exchangeability of

Z

and

Z_{i}

gives

E ℓ (H_{D}, Z) = \frac{1}{n} \sum_{i = 1}^{n} E ℓ (H_{D_{i} (Z)}, Z_{i}) .

The datasets

D_{i} (Z)

and

D

differ in one coordinate, so stability gives

E ℓ (H_{D_{i} (Z)}, Z_{i}) \leq E ℓ (H_{D}, Z_{i}) + β .

Averaging over

i

and subtracting the empirical loss proves

E F (D) \leq β .

Solved by gpt-5.6-sol high.

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