This is a German wiki article but the math is probably understandable anyways. There is an example of branch and bound and the concept of dominance is shown. It is applied to an example and they come up with $x_1/2 - x_2$ as a term that should show dominance over the realm of all possible parameters. However, it is not explained at all where this term comes from. I would understand if $x_1$ were set equal to 2 but it is not.
Can anyone help? Here is the link: https://de.wikipedia.org/wiki/Branch-and-Bound#Dominanz