The AI Productivity Index (APEX). - Bertie Vidgen; Abby Fennelly; Evan Pinnix; Chirag Mahapatra; Zach Richards; Austin Bridges; Calix Huang; Ben Hunsberger; Fez Zafar; Brendan Foody; Dominic Barton; Cass R. Sunstein; et al. (CoRR, 2025)