I came across this sentence in the Wikipedia article on Shell short, which states that If the file is then k-sorted for some smaller integer k, then the file remains h-sorted.I've seen this claim in other texts like Lafore as well, but I can't wrap my head around why it's true.  Of course I can't find a counterexample, but why must it be so?
Why does decreasing the gap size in Shell sort never undo previous sorts?
algorithms;sorting