Öztop, H., D. Kizilay, and Z. A. Çil. “Mathematical Models for the Periodic Vehicle Routing Problem With Time Windows and Time Spread Constraints”. An International Journal of Optimization and Control: Theories & Applications (IJOCTA), vol. 11, no. 1, Sept. 2020, pp. 10-23, doi:10.11121/ijocta.01.2021.00899.