III. d(f,g) ≤ α(f,h)d(f,h) + β(h,g)d(h,g) ꓯ f,g,h ε Ɀ. Then
d is called a double controlled metric and (Ɀ,d) is a double controlled metric space.
Example2.7 [3]. Let Ɀ = [0,∞) and a function define d : Ɀ х Ɀ→ [0,∞) such that
d(f,g) = 0, if x = y,
=
1
𝑓
, if f ≥ 1 and g ε [0,1)
=
1
𝑔
, if g ≥ 1 and f ε [0,1)
= 1, otherwise
and the controlled functions α, β : Ɀ х Ɀ → [1,∞) by
α(f,g) = f, if f, g ≥ 1 and β(f,g) = 1, if f,g < 1
= 1, otherwise = max{f,g}, otherwise.
Then d is a double controlled metric and (Ɀ,d) is a double controlled metric space.
Singh et al. introduce a generalization of the controlled metric space by introducing three control functions.
Definition 2.8 [6] Let Ɀ ≠ Ф, d : Ɀ х Ɀ х Ɀ → [0,∞)be a non- negative function and α,β,γ : Ɀ х Ɀ х Ɀ → [1,∞)
be three control functions satisfying the following conditions:
I. d(f,g,h) = 0 if at least two of the three points are the same.
II. For f,g ε Ɀ such that f ≠ g there exist a point h ε Ɀ such that d(f,g,h) ≠ 0.
III. For f,g,h ε Ɀ, d(f,g,h) = d(f,h,g) = d(g,h,f) = d(h,f,g) = d(g,f,h) = d(h,g,f)
IV. For f,g,h,a ε Ɀ, d(f,g,h) ≤ α(f,g,a)d(f,g,a) + β(g,h,a)d(g,h,a) + γ(h,f,a)d(h,f,a).
Then d is called controlled metric and (Ɀ,d) is a controlled metric space.
Example 2.9 Let Ɀ = (0,1) and d : Ɀ х Ɀ х Ɀ → [0,∞)is defined by
d(f,g,h) = 0 , if at least two of the three points are the same.
= μ(f,g,h) e
1/2│f-g│+ 3/8│g-h│+ 1/8│h-f│
otherwise. For
continuous α(f,g,h) ≥ 0 symmetric in f,g,h. If suffices to only verify property
(IV) of definition 2.8.Which typically in such generalized setting takes a form like.
For all f,g,h,t ε Ɀ .
d(f,g,h) = α(f,g,t) d(f,g,t) + β(g,h,t) d(g,h,t) + γ(h,f,t) d(h,f,t)
for some control function α,β,γ : Ɀ х Ɀ х Ɀ → [1,∞).
Using Jensen
,
s inequality applied of the convex function f(x) = e
x
along with modulus power │ a│.
d(f,g,h) = μ(f,g,h) e
1/2│f-g│+ 3/8│g-h│+ 1/8│h-f│
≤
μ(f,g,h) [1/2 e
│f-g│
+
3/8 e
│g-h│
+1/8 e
│h-f│
]