Shape of Memory: a Geometric Analysis of Machine Unlearning in Second-Order Optimizers

arXiv cs.LG / 4/28/2026

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Key Points

  • The paper argues that existing definitions of machine unlearning do not adequately account for how second-order optimizers behave during data deletion.
  • It compares first-order and second-order learners under unlearning scenarios that vary eigen-decomposition structure to model stored “memory” in the loss.
  • While both types can match the ideal counterfactual in performance and gradients, second-order optimization shows large volatility in its optimizer state.
  • The authors find that residual (supposedly deleted) information may persist in second-order optimizer states and is not captured by first-order gradient-based checks.
  • Stability and effective information erasure are recovered only when state perturbations are controlled such that geometric information (memory) is explicitly removed.

Abstract

We argue that current definitions of machine unlearning are underspecified for second-order optimizers. We compare first-order and second-order learners for their ability to handle the data deletion task with varying degrees of eigendecomposition to mimic the loss model memory. While both first and second-order methods realign with the ideal counterfactul in terms of performance and gradient, the second-order optimizer shows significant volatility in the optimizer state. This indicates residual information, supposedly deleted, that isn't detectable by first-order analysis. Various eigendecay treatments show that stability and information loss is regained only under controlled state pertubation where geometric information (or memory) is erased.