M: is a bibliographic data frame obtained by the converting function convert2df. It is a data matrix with cases corresponding to manuscripts and variables to Field Tag in the original SCOPUS and Clarivate Analytics WoS file.
breaks: is a numeric vector of two or more unique cut points.
k: is an integer value giving the number of intervals into which the data frame is to be cut. k is used only in case breaks argument is not provided. The default is k = 5.
Returns
the value returned from split is a list containing the data frames for each sub-period.