POLAR: Policy-based Layerwise Reinforcement Learning Method for Stealthy Backdoor Attacks in Federated Learning. - Kuai Yu; Xiaoyu Wu; Peishen Yan; Qingqian Yang; Linshan Jiang; Hao Wang 022; Yang Hua 001; Tao Song 003; Haibing Guan (CoRR, 2025)