Mitigating Forgetting in LLM Fine-Tuning via Low-Perplexity Token Learning - Wu, Chao-Chung; Tam, Zhi Rui; Lin, Chieh-Yen; Chen, Yun-Nung (Vivian); Sun, Shao-Hua; Lee, Hung-yi (Advances in Neural Information Processing Systems, 2025)