Z.M. Ma, B.Q. Hu / Fuzzy Sets and Systems 247 (2014) 92–107 95
The following alternative definitions of filters are useful and give us an inspiration.
Definition 2.8. (See [3,10–12,20,22,30,35–38].)
Let F beafilterofL. Then F is called
• an
implicative filter if x → x
2
∈ F for all x ∈ L ,
• a
regular filter if ¬¬x → x ∈ F for all x ∈ L,
• a
fantastic filter if (y → x) →{[(x → y) → y]→x}∈F for all x,y ∈ L,
• a
Boolean filter if x ∨¬x ∈ F for all x ∈ L,
• a
prime filter if F is either x → y ∈ F or y → x ∈ F for all x,y ∈ L,
• a
prime filter of the second kind if F is proper such that x ∨ y ∈ F implies x ∈ F or y ∈ F for all x,y ∈ L,
• a
prime filter of the third kind if (x → y) ∨ (y → x) ∈ F for all x,y ∈ L, equivalently, x → (y ∨ z) ∈ F implies
(x → y) ∨ (x → z) for all x,y,z ∈ L [25].
Note 2.9. An
implicative filter is also called a G-filter [34,38] or a positive implicative filter [10,22,23] and a Boolean
filter is also called a positive implicative filter [12,20] or an implicative filter [30].
Lemma 2.10. (See [12,35,37,38].)
Let F beafilterofL. Then the following assertions are equivalent, respectively
for all x,y,z ∈ L.
• (I1) F is
an implicative filter.
(I2) z → (y → x) ∈ F implies (z → y) → (z → x) ∈ F .
(I3) y → (y → x) ∈ F implies y → x ∈ F .
• (R1) F is
a regular filter.
(R2) ¬x →¬y ∈ F implies y → x ∈ F .
(R3) ¬x → y ∈ F implies ¬y → x ∈ F .
• (F1) F is
a fantastic filter.
(F2) y → x ∈ F implies [(x → y) → y]→x ∈ F .
(F3) [(x → y) → y]→[(y → x) → x]∈F .
• (B1) F is
a Boolean filter.
(B2) (¬x → x) → x ∈ F .
(B3) (x → y) → x ∈ F implies x ∈ F .
Definition 2.11. (See [10].)
Let S be a set and C be a subset of its powerset P(S). We say that C satisfies the extension
property if and only if C is closed under supersets that are filters, i.e. S
2
∈ C whenever S
1
∈ C and S
2
is a filter with
S
1
⊆ S
2
⊆ S.
In [10],
it was pointed out that the filters defined in Definition 2.8 satisfy the extension property except for prime
filters of the second kind.
3. I -filters and their characterizations
In this section, we will propose a formal definition and characterizations for several classes of filters.
Definition 3.1. Let F b
eafilterofL. For all x,y ∈ L, if there exist terms t(x,y),t
(x, y) of L such that t
(x, y)
t(x,y) and t(x,y) → t
(x, y) ∈ F , then F is called an I -filter w.r.t. t(x,y) → t
(x, y), and I-filter for short.
I-filter
can be considered as a formal definition for some types of filters such as implicative filters, regular filters,
fantastic filters, Boolean filters and prime filters of the third kind in Definition 2.8 when t(x,y) → t
(x, y) is x → x
2
,
¬¬x → x, (y → x) →{[(x → y) → y]→x},1→ (x ∨¬x) and 1 →[(x → y) ∨ (y → x)], respectively. Although
it is identical with that of filter defined by identity form [20], it possesses a specific form in which the basic operation
“residuum” is included, and provides us with a meaningful method to characterize these filters and their corresponding
algebras.