  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.
