The First Few Tokens Are All You Need: An Efficient and Effective Unsupervised Prefix Fine-Tuning Method for Reasoning Models - Ji, Ke; Xu, Jiahao; Liang, Tian; Liu, Qiuzhi; He, Zhiwei; Liu, Xiaoyuan; Chen, Xingyu; Chen, Junying; Wang, Benyou; Tu, Zhaopeng; Mi, Haitao; Yu, Dong (Advances in Neural Information Processing Systems, 2025)