Title: F-index of graphs based on four operations related to the lexicographic product,
Abstract: The forgotten topological index or F-index of a graph is defined as the sum
of cubes of the degree of all the vertices of the graph. In this paper we study
the F-index of four operations related to the lexicographic product on graphs
which were introduced by Sarala et al. [D. Sarala, H. Deng, S.K. Ayyaswamya and
S. Balachandrana, The Zagreb indices of graphs based on four new operations
related to the lexicographic product, \textit{Applied Mathematics and
Computation}, 309 (2017) 156--169.].