Dragomir, S. S., and N. S. Barnett. “An Ostrowski Type Inequality for Mappings Whose Second Derivatives Are Bounded and Applications”. The Journal of the Indian Mathematical Society, vol. 66, no. 1-4, Dec. 1999, pp. 237-45, https://informaticsjournals.co.in/index.php/jims/article/view/22008.