Ordering Fréchet–Urysohn filters

A free filter FF on the natural numbers ω is called Fréchet–Urysohn (FU  ) if the space ω∪{F}ω∪{F} with just one accumulation point is a Fréchet–Urysohn space, where the neighborhoods of FF are of the form {F}∪F{F}∪F for F∈FF∈F. The Fréchet filter and the countable FAN-filter are the known examples of FU  -filters, but we know that there are 2c2c-many pairwise non-equivalent FU-filters. In this article, we shall order this kind of filters by using the Rudin–Keisler order of filters.