Essential concepts of digital topology\\ (digital $k$-connectivity and $k$-adjacencis for digital products)

The paper refers to several concepts which are essential to studying digital objects from the viewpoint of digital topology: digital $k$-connectivity or digital $k$-adjacency, $C$-compatible and normal $k$-adjacency for a digital product. Since L. Boxer has often mentioned the origins of these concepts in an inaccurate way, we discuss something incorrectly cited or mentioned in Boxer’s papers according to the facts.