slin line的不存在性。Ronald Jensen证明了CH并不推出Suslin line的存在性。假设不可达基数的相容性之前提下，Kurepa tree的存在性与ZFC独立。Existence of a partition of the序数formula_4 into two colors with no monochromatic uncountable sequentially closed subset is independent of ZFC, ZFC + CH, and ZF