While I was looking through a textbook (the name of the book and the author is indicated below), I came across the following symbols which I could not understand.
Source: Introductory Notes in Discrete Mathematics by Marcel B. Finan
I indicated the symbols with a red circle and an ellipse. The asterisk normally means multiplication. But it is used in a different sense in the solution.
Also, what is the meaning of the black-filled square? I guess that it signifies the end of solution. What do you think?