Dunbar, D., Bern, Z., Dunbar, D., & Shimada, T. (1993). String-based methods in perturbative gravity. Physics Letters B, 312(3), pp. 277-284. doi:10.1016/0370-2693(93)91081-W

Dunbar, David, Z. Bern, D.C Dunbar, and T. Shimada. "String-based Methods in Perturbative Gravity." Physics Letters B 312, no. 3 (1993): 277-284.

Dunbar, David, Z. Bern, D.C Dunbar, and T. Shimada. "String-based Methods in Perturbative Gravity." Physics Letters B 312.3 (1993): 277-284.