Beckmann, A., & Buss, S. (2017). The NP Search Problems of Frege and Extended Frege Proofs. ACM Transactions on Computational Logic, 18(2), pp. 1-19. doi:10.1145/3060145

Beckmann, Arnold, and Sam Buss. "The NP Search Problems of Frege and Extended Frege Proofs." ACM Transactions On Computational Logic 18, no. 2 (2017): 1-19.

Beckmann, Arnold, and Sam Buss. "The NP Search Problems of Frege and Extended Frege Proofs." ACM Transactions On Computational Logic 18.2 (2017): 1-19.