Ordering Fréchet–Urysohn filters

A free filter F on the natural numbers ω is called Fréchet–Urysohn (FU) if the space ω ∪ { F } with just one accumulation point is a Fréchet–Urysohn space, where the neighborhoods of F are of the form { F } ∪ F for F ∈ F . The Fréchet filter and the countable FAN-filter are the known examples of FU-filters, but we know that there are 2 c -many pairwise non-equivalent FU-filters. In this article, we shall order this kind of filters by using the Rudin–Keisler order of filters.