How Much Rank Does LoRA Need? Rank-Error Bounds for Transformer Attention

arXiv:2608.26052v1 Announce Type: cross Abstract: Choosing the rank of a low-rank adaptation (LoRA) update is usually an empirical task. In this paper, we provide a task-dependent theory of the approximation error achievable at each LoRA rank for Transformer attention. We fix a pretrained attention head, a target attention function, and a distribution over inputs from the downstream task, and bound the smallest expected Kullback--Leibler (KL) error achievable by a rank-r query LoRA update. When target attention probabilities are bounded away from zero, we prove a lower bound of the error proportional to psi(|d|_2), where d is the difference between candidate and target attention scores and psi(t)=min{t^2,t}. We also prove an unconditional upper bound min{|d|_2^2/4,sqrt2|d|_2}. Under explicit realizability, geometry, and moment conditions, we then bound the best rank-r error between an explicit multiple of psi(sqrt{T_r}) and min{T_r/4,sqrt{2T_r}}, where T_r is the downstream-weighted tail energy of the target update. We also provide target-Fisher bounds when candidate scores remain within a fixed range of the target scores, and an unrestricted lower bound when a subset of tokens carries most of the probability mass. These spectral bounds describe finite-score approximation. We then construct explicit families in which softmax saturation makes the rank required to match the attention function strictly smaller than the rank required to match the finite logits. Finally, we extend the analysis to fused multi-head LoRA and joint query/key updates, exposing the effects of rank sharing and query/key factorization constraints.

Source: arXiv cs.CL | 2026-08-27

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