Is there an easy formal proof for this: http://yetanothermathprogrammingconsultant.blogspot.com/2011/10/special-case-of-integer-cuts.html? I think I derived it once. The more general cut is derived here: http://yetanothermathprogrammingconsultant.blogspot.com/2011/10/integer-cuts.html.
Update. Here are my dabblings. First we go back to the derivation shown here and take the final formulation for the cut. From there we can do: