I am reading this paper about interleavers for turbo code design, and when it describes the so called block interleavers, it says that
To obtain a block interleaver function it is necessary to factorize its length:
where $X$ and $Y$ are closed natural numbers.
I have been looking around what the word closed means for natural numbers, but I have only found definitions about closure of sets, and not individual numbers. I am wondering what could such thing mean in this context.