To provide you with a proper reference for Bertola and Hartley, I would need more specific information about the work you are referring to, such as the title of the paper or book, the publication year, the journal or publisher, and any relevant page numbers.
However, if you are looking for a common reference, one influential paper by Bertola and Hartley is:
Bertola, G., & Hartley, J. (1999). The Role of the Financial Sector in the Real Economy. In The Handbook of Macroeconomics (Vol. 1, pp. 215-270). Elsevier Science.
Please ensure to verify this citation based on your required citation style (APA, MLA, Chicago, etc.) and modify it accordingly! If this is not the correct reference, please provide additional details, and I can assist further!