Spectrally Transformed Kernel Regression
Authors: Runtian Zhai,Rattana Pukdee,Roger Jin,Maria Florina Balcan,Pradeep Kumar Ravikumar
Presentation: ICLR 2024,Spotlight

Abstract: Unlabeled data is a key component of modern machine learning. In general, the role
of unlabeled data is to impose a form of smoothness, usually from the similarity
information encoded in a base kernel, such as the ϵ-neighbor kernel or the adjacency
matrix of a graph. This work revisits the classical idea of spectrally transformed
kernel regression (STKR), and provides a new class of general and scalable STKR
estimators able to leverage unlabeled data. Intuitively, via spectral transformation,
STKR exploits the data distribution for which unlabeled data can provide additional
information. First, we show that STKR is a principled and general approach,
by characterizing a universal type of “target smoothness”, and proving that any
sufficiently smooth function can be learned by STKR. Second, we provide scalable
STKR implementations for the inductive setting and a general transformation
function, while prior work is mostly limited to the transductive setting. Third, we
derive statistical guarantees for two scenarios: STKR with a known polynomial
transformation, and STKR with kernel PCA when the transformation is unknown.
Overall, we believe that this work helps deepen our understanding of how to work
with unlabeled data, and its generality makes it easier to inspire new methods.