Anomalies as Residuals: Cauchy Displacement Networks

Abstract

Neighbourhood-based detectors such as KNN, LOF and LUNAR remain among the most accurate methods for tabular anomaly detection, but they compare every query with the stored training set, so their inference cost and memory grow with the data and the data must be kept at deployment. We ask whether the neighbourhood geometry these detectors exploit can be compiled into a small network that scores a query with a single forward pass. We propose the Cauchy Displacement Network (CDN), which regresses each normal point onto the mean of its $k$ nearest training neighbours with a single-hidden-layer network of Cauchy activations and scores a query by the length of its predicted displacement. This displacement is an empirical mean-shift vector, whose fixed-radius population counterpart locally approximates a scaled gradient of the log density. We show that Cauchy units exactly represent the score fields of a structured family of Student-$t$ expert densities, and establish a constructive approximation bound for the corresponding population neighbourhood-mean map using a single-hidden-layer Cauchy network. We further show that the bounded network output guarantees asymptotically linear residual-score growth along every fixed direction. On 46 benchmark datasets against 28 baselines, CDN obtains the best mean rank under both AUC-ROC and AUC-PR, while scoring $10^4$ queries in a few milliseconds on a single CPU thread, up to three orders of magnitude faster than neighbourhood-based detectors, with a model whose size does not depend on the training set.