In set theory, Silver machines are devices used for bypassing the use of fine structure in proofs of statements holding in L. They were invented by set theorist Jack Silver as a means of proving global square holds in the constructible universe.
An ordinal
α
is *definable from a class of ordinals X if and only if there is a formula
ϕ
(
μ
0
,
μ
1
,
…
,
μ
n
)
and
∃
β
1
,
…
,
β
n
,
γ
∈
X
such that
α
is the unique ordinal for which
⊨
L
γ
ϕ
(
α
∘
,
β
1
∘
,
…
,
β
n
∘
)
where for all
α
we define
α
∘
to be the name for
α
within
L
γ
.
A structure
⟨
X
,
<
,
(
h
i
)
i
<
ω
⟩
is eligible if and only if:
-
X
⊆
O
n
.
- < is the ordering on On restricted to X.
-
∀
i
,
h
i
is a partial function from
X
k
(
i
)
to X, for some integer k(i).
If
N
=
⟨
X
,
<
,
(
h
i
)
i
<
ω
⟩
is an eligible structure then
N
λ
is defined to be as before but with all occurrences of X replaced with
X
∩
λ
.
Let
N
1
,
N
2
be two eligible structures which have the same function k. Then we say
N
1
◃
N
2
if
∀
i
∈
ω
and
∀
x
1
,
…
,
x
k
(
i
)
∈
X
1
we have:
h
i
1
(
x
1
,
…
,
x
k
(
i
)
)
≅
h
i
2
(
x
1
,
…
,
x
k
(
i
)
)
A Silver machine is an eligible structure of the form
M
=
⟨
O
n
,
<
,
(
h
i
)
i
<
ω
⟩
which satisfies the following conditions:
Condensation principle. If
N
◃
M
λ
then there is an
α
such that
N
≅
M
α
.
Finiteness principle. For each
λ
there is a finite set
H
⊆
λ
such that for any set
A
⊆
λ
+
1
we have
M
λ
+
1
[
A
]
⊆
M
λ
[
(
A
∩
λ
)
∪
H
]
∪
{
λ
}
Skolem property. If
α
is *definable from the set
X
⊆
O
n
, then
α
∈
M
[
X
]
; moreover there is an ordinal
λ
<
[
s
u
p
(
X
)
∪
α
]
+
, uniformly
Σ
1
definable from
X
∪
{
α
}
, such that
α
∈
M
λ
[
X
]
.