Sparse but Critical: A Token-Level Analysis of Distributional Shifts in RLVR Fine-Tuning of LLMs - Meng, Haoming; Huang, Kexin; Wei, Shaohang; Ma, Chiyu; Yang, Shuo; wang, xue; Wang, Guoyin; Ding, Bolin; Zhou, Jingren (International Conference on Learning Representations, 2026)