Gravitational anomaly and Hawking radiation of apparent horizon in FRW universe Ran Li, Ji-Rong Ren and Shao-Wen Wei Eur. Phys. J. C, 62 3 (2009) 455-458 Published online: 26 June 2009 DOI: 10.1140/epjc/s10052-009-1081-4