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The formula given is not actually in Skolem normal form since it contains an existence quantifier.
It should be:
F
S
=
∀
y
∀
z
[
R
(
y
,
f
x
(
y
)
)
∧
¬
S
(
f
x
(
y
)
,
z
)
]
.
{\displaystyle F^{S}=\forall y\forall z[R(y,f_{x}(y))\wedge \neg S(f_{x}(y),z)].}
Step by step:
F
:=
∀
y
∃
x
[
R
(
y
,
x
)
∧
¬
∃
z
S
(
x
,
z
)
]
{\displaystyle F:=\forall y\exists x[R(y,x)\wedge \neg \exists zS(x,z)]}
(original formula)
F
:=
∀
y
∃
x
[
R
(
y
,
x
)
∧
∀
z
¬
S
(
x
,
z
)
]
{\displaystyle F:=\forall y\exists x[R(y,x)\wedge \forall z\neg S(x,z)]}
(replace negated existence with all quantor with negated statement)
F
:=
∀
y
∃
x
∀
z
[
R
(
y
,
x
)
∧
¬
S
(
x
,
z
)
]
{\displaystyle F:=\forall y\exists x\forall z[R(y,x)\wedge \neg S(x,z)]}
(move all quantifier to begin to formula to get prenex normal form)
F
:=
∀
y
∀
z
[
R
(
y
,
f
x
(
y
)
)
∧
¬
S
(
f
x
(
y
)
,
z
)
]
{\displaystyle F:=\forall y\forall z[R(y,f_{x}(y))\wedge \neg S(f_{x}(y),z)]}
(replace variable bound by existence quantifier with function depending on preceding all quantors)
79.212.0.144 (talk ) 18:22, 12 April 2016 (UTC) [ reply ]