In mathematics, the Rabinowitsch trick, introduced by George Yuri Rainich and published under his original name Rabinowitsch (1929), is a short way of proving the general case of the Hilbert Nullstellensatz from an easier special case (the so-called weak Nullstellensatz), by introducing an extra variable.
The Rabinowitsch trick goes as follows. Let K be an algebraically closed field. Suppose the polynomial f in K[x1,...xn] vanishes whenever all polynomials f1,....,fm vanish. Then the polynomials f1,....,fm, 1 − x0f have no common zeros (where we have introduced a new variable x0), so by the weak Nullstellensatz for K[x0, ..., xn] they generate the unit ideal of K[x0 ,..., xn]. Spelt out, this means there are polynomials
g
0
,
g
1
,
…
,
g
m
∈
K
[
x
0
,
x
1
,
…
,
x
n
]
such that
1
=
g
0
(
x
0
,
x
1
,
…
,
x
n
)
(
1
−
x
0
f
(
x
1
,
…
,
x
n
)
)
+
∑
i
=
1
m
g
i
(
x
0
,
x
1
,
…
,
x
n
)
f
i
(
x
1
,
…
,
x
n
)
as an equality of elements of the polynomial ring
K
[
x
0
,
x
1
,
…
,
x
n
]
. Since
x
0
,
x
1
,
…
,
x
n
are free variables, this equality continues to hold if expressions are substituted for some of the variables; in particular, it follows from substituting
x
0
=
1
/
f
(
x
1
,
…
,
x
n
)
that
1
=
∑
i
=
1
m
g
i
(
1
/
f
(
x
1
,
…
,
x
n
)
,
x
1
,
…
,
x
n
)
f
i
(
x
1
,
…
,
x
n
)
as elements of the field of rational functions
K
(
x
1
,
…
,
x
n
)
, the field of fractions of the polynomial ring
K
[
x
1
,
…
,
x
n
]
. Moreover, the only expressions that occur in the denominators of the right hand side are f and powers of f, so rewriting that right hand side to have a common denominator results in an equality on the form
1
=
∑
i
=
1
m
h
i
(
x
1
,
…
,
x
n
)
f
i
(
x
1
,
…
,
x
n
)
f
(
x
1
,
…
,
x
n
)
r
for some natural number r and polynomials
h
1
,
…
,
h
m
∈
K
[
x
1
,
…
,
x
n
]
. Hence
f
(
x
1
,
…
,
x
n
)
r
=
∑
i
=
1
m
h
i
(
x
1
,
…
,
x
n
)
f
i
(
x
1
,
…
,
x
n
)
,
which literally states that
f
r
lies in the ideal generated by f1,....,fm. This is the full version of the Nullstellensatz for K[x1,...,xn].