Projective hierarchy explained
In the mathematical field of descriptive set theory, a subset
of a
Polish space
is
projective if it is
for some positive integer
. Here
is
if
is
analytic
if the
complement of
,
, is
if there is a Polish space
and a
subset
such that
is the
projection of
onto
; that is,
A=\{x\inX\mid\existsy\inY:(x,y)\inC\}.
The choice of the Polish space
in the third clause above is not very important; it could be replaced in the definition by a fixed
uncountable Polish space, say
Baire space or
Cantor space or the
real line.
Relationship to the analytical hierarchy
There is a close relationship between the relativized analytical hierarchy on subsets of Baire space (denoted by lightface letters
and
) and the projective hierarchy on subsets of Baire space (denoted by boldface letters
and
). Not every
subset of Baire space is
. It is true, however, that if a subset
X of Baire space is
then there is a set of
natural numbers
A such that
X is
. A similar statement holds for
sets. Thus the sets classified by the projective hierarchy are exactly the sets classified by the relativized version of the analytical hierarchy. This relationship is important in
effective descriptive set theory. Stated in terms of definability, a set of reals is projective iff it is definable in the language of
second-order arithmetic from some real parameter.
[1] A similar relationship between the projective hierarchy and the relativized analytical hierarchy holds for subsets of Cantor space and, more generally, subsets of any effective Polish space.
See also
Notes and References
- J. Steel, "What is... a Woodin cardinal?". Notices of the American Mathematical Society vol. 54, no. 9 (2007), p.1147.