Online Pairwise Learning Algorithms with Kernels
This work addresses the challenge of online pairwise learning for tasks like ranking and AUC maximization by removing constraints on iterates or strong convexity, offering a more flexible approach.
The authors tackled the problem of online pairwise learning in an unconstrained reproducing kernel Hilbert space (RKHS) by proposing the OPERA algorithm, which achieves almost sure convergence for the last iterate without distributional assumptions and provides explicit convergence rates with polynomially decaying step sizes.
Pairwise learning usually refers to a learning task which involves a loss function depending on pairs of examples, among which most notable ones include ranking, metric learning and AUC maximization. In this paper, we study an online algorithm for pairwise learning with a least-square loss function in an unconstrained setting of a reproducing kernel Hilbert space (RKHS), which we refer to as the Online Pairwise lEaRning Algorithm (OPERA). In contrast to existing works \cite{Kar,Wang} which require that the iterates are restricted to a bounded domain or the loss function is strongly-convex, OPERA is associated with a non-strongly convex objective function and learns the target function in an unconstrained RKHS. Specifically, we establish a general theorem which guarantees the almost surely convergence for the last iterate of OPERA without any assumptions on the underlying distribution. Explicit convergence rates are derived under the condition of polynomially decaying step sizes. We also establish an interesting property for a family of widely-used kernels in the setting of pairwise learning and illustrate the above convergence results using such kernels. Our methodology mainly depends on the characterization of RKHSs using its associated integral operators and probability inequalities for random variables with values in a Hilbert space.