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Abstract
We show that a global holomorphic section of restricted to a closed complex subspace
has an interpolant if and only if it satisfies a set of moment conditions that involves a residue current associated with a locally free resolution of
. When X is a finite set of points in
this can be interpreted as a set of linear conditions that a function on X has to satisfy in order to have a polynomial interpolant of degree at most d.
1. Introduction
Let be a subvariety or complex subspace whose underlying space,
, is a finite set of points
. Let g be a holomorphic function on X, i.e. a global holomorphic section of
, and let
be a polynomial. We say that G interpolates g if the pull-back of G to X equals g, i.e.
.
If X is reduced, then a holomorphic function g on X is just a function from to
, and we have that G interpolates g if
for each
. In the univariate case this is referred to as Lagrange interpolation. If X is not reduced, then at each point
, G also has to satisfy some conditions on its derivatives. In the univariate case this is referred to as Hermite interpolation, see Example 4.3.
The motivating question for this note is the following. What are the necessary and sufficient conditions on g for the existence of an interpolant of degree at most d?
Let denote the vector space of holomorphic functions on X, i.e.
. Since the set of holomorphic functions on X that have an interpolant of degree at most d is a linear subspace of
, we have that a function
has an interpolant of degree at most d if and only if it satisfies a finite set of linear conditions. In this note we will show how these linear conditions can be explicitly realized as a set of moment conditions that involves a so-called residue current associated with a locally free resolution of
.
Recall that since is a finite set of points, X can be viewed as a closed complex subspace of
, and we have that polynomials of degree at most d on
naturally correspond to global holomorphic sections of the line bundle
via d-homogenization. This motivates the following more general notion of interpolation that we shall consider in this note. Let
be a closed complex subspace of arbitrary dimension. Let Φ and φ be global holomorphic sections of
and
, respectively. We say that Φ interpolates φ if
.
From a minimal graded free resolution of the homogeneous coordinate ring of X, , we obtain a locally free resolution of
of the form
(1)
(1)
where
, see [Citation1] and [Citation2, Section 6]. The
are referred to as the graded Betti numbers of
. We equip the
with the natural Hermitian metrics. In [Citation2], Andersson and Wulcan showed that with (Equation1
(1)
(1) ), one can associate a residue current R that generalizes the classical Coleff–Herrera product [Citation3], see Section 2. It can be written as
, where each
is an
-valued
-current. In [Citation4], the same authors proved a result which as a special case gives a cohomological condition in terms of the current R for when Φ interpolates φ. In Section 3 we will show that in our setting this condition amounts to the following set of moment conditions.
Theorem 1.1
Let be a closed complex subspace, and let R be the residue current associated with (Equation1
(1)
(1) ). Moreover, let ω be a nonvanishing holomorphic
-valued n-form. A global holomorphic section φ of
has an interpolant if and only if for each ℓ it holds that
(2)
(2)
for all global holomorphic sections h of
.
Recall that the interpolation degree of X is defined as
In particular, if
is a finite set of points in
, then the interpolation degree of X is the smallest number d such that any
has an interpolant of degree at most d. Define
(3)
(3)
(We use the convention that the supremum of the empty set is
.) As a consequence of Theorem 1.1 we get the following bound of the interpolation degree.
Corollary 1.2
Let be a closed complex subspace with homogeneous coordinate ring
. The interpolation degree of X is less than or equal to
.
It can be shown by purely algebraic means that the interpolation degree of X is in fact equal to , see e.g. [Citation5, Corollary 1.6]. If
consists of a finite set of points, then it can be shown that
is equal to the Castelnuovo–Mumford regularity of
, see [Citation1, Exercise 4E.5], and the statement in this case is Theorem 4.1 in [Citation1].
In Section 4 we will consider the case when is a finite set of points in
. In this case the corresponding versions of Theorem 1.1 and Corollary 1.2 first appeared in [Citation6]. We will consider some examples where we explicitly write down the conditions for when
has an interpolant of degree at most d. In particular, we will obtain the precise conditions for when the Hermite interpolation problem has a solution.
2. Residue currents
Let f be a holomorphic function in an open set in . Let ξ be a smooth
-form with compact support. In [Citation7], using Hironaka's desingularization theorem, Herrera and Lieberman proved that the limit
(4)
(4)
exists. Thus (Equation4
(4)
(4) ) defines a current known as the principal value current, which is denoted by
. The residue current
of f is the
-current
. It is easy to see that
has its support on
, and that it satisfies the following duality principle: A holomorphic function Φ belongs to the ideal
if and only if
.
Example 2.1
Let . We have that the action of
on a test form
is given by
(5)
(5)
2.1. Residue currents associated with generically exact complexes
We will now consider a generalization of the above construction due to Andersson and Wulcan. Consider a generically exact complex of Hermitian holomorphic vector bundles over a complex manifold Y of dimension n,
(6)
(6)
i.e. a complex that is exact outside an analytic variety
of positive codimension. The vector bundle
has a natural superbundle structure, i.e. a
-grading,
, where
and
, which we shall refer to as the subspaces of even and odd elements, respectively. This induces a
-grading on the sheaf of E-valued currents
; if
is an E-valued current, where ω is a current and ξ is a smooth section of E, then the degree of
is the sum of the degree of ξ and the current degree of ω modulo 2.
We say that an endomorphism on E is even (resp. odd) if it preserves (resp. switches) the degree. If α is a smooth section of , then it defines a map on
via
where ω is a current and ξ is a smooth section of E. In particular, the map
defines an odd map on
. We define an odd map on
,
, which, since f and
anti-commute, satisfies
. The map ∇ extends to an odd map on
via Leibniz's rule,
In [Citation2], Andersson and Wulcan constructed
-valued currents
and
where
and
are
-valued currents of bidegree
and
, respectively, which satisfy
(7)
(7)
The current R is referred to as the residue current associated with (Equation6
(6)
(6) ) and it has its support on Z.
Suppose that the complex of locally free sheaves corresponding to (Equation6(6)
(6) ),
(8)
(8)
is exact. When the
are equipped with Hermitian metrics, we shall refer to (Equation8
(8)
(8) ) as a Hermitian resolution of the sheaf
. In this case it holds that
if
, and henceforth we shall write
for
. Moreover, we have that R satisfies the following properties:
Duality principle: A holomorphic section Φ of belongs to
if and only if
.
Dimension principle: If , then
.
Note that the second equality in (Equation7(7)
(7) ) is equivalent to
(9)
(9)
(10)
(10)
(11)
(11)
Let
be a closed complex subspace with ideal sheaf
, and suppose that
, which we identify with
, has a Hermitian resolution of the form
(12)
(12)
cf.(Equation8
(8)
(8) ) where
is the trivial line bundle. For the associated residue current
, we can view each
as an
-valued
-current. Since
, we have that
if and only if
by the duality principle. More generally, let
be a holomorphic line bundle. If we equip L with a Hermitian metric, then we obtain a Hermitian resolution of
by tensoring (Equation12
(12)
(12) ) with L, and we have that R is the associated residue current with this resolution as well.
2.2. The Coleff–Herrera product
Let be a holomorphic mapping such that
has codimension p. In [Citation3] Coleff and Herrera gave meaning to the product
(13)
(13)
which is known as the Coleff–Herrera product. In particular, if each
only depends on
, then (Equation13
(13)
(13) ) is just the tensor product of the one-variable currents
described above. The current
is
-closed, has support
, and is anti-commuting in the
. Moreover,
satisfies the duality principle, i.e.
if and only if
, where
is the ideal sheaf generated by f.
Let be a holomorphic Hermitian vector bundle of rank p, and let f be a holomorphic section of the dual bundle
. Let
, and define
as interior multiplication by f. This gives a generically exact complex (Equation6
(6)
(6) ). Suppose
in some local holomorphic frame
for
. If
, then the corresponding complex of sheaves is a Hermitian resolution of
known as the Koszul complex, and it was proven in [Citation8] that the associated residue current is given by
.
2.3. A comparison formula for residue currents
We have the following comparison formula for residue currents, see Theorem 1.3 and Corollary 4.7 in [Citation9]. Let be complex subspaces of codimension p of Y. Suppose that there exist Hermitian resolutions of length p of
and
, respectively, and let R and
be the associated residue currents. Moreover, suppose that there exists a map of complexes
Then .
3. Interpolation and residue currents
Let Y be a complex manifold of dimension n, and let be a closed complex subspace. Let
be a holomorphic line bundle, and let Φ and φ be global holomorphic sections of L and
, respectively. We say that Φ interpolates φ if
.
Suppose that there exists a Hermitian resolution of of the form (Equation12
(12)
(12) ), and let R denote the associated residue current. For each point
there is a neighbourhood
and a holomorphic section
of L such that
on
. We define the current
on Y locally as
. This is well-defined since if
is another section such that
, then
by the duality principle.
We have the following result which follows immediately as a special case of Lemma 4.5 (ii) in [Citation4].
Lemma 3.1
Let Φ and φ be global holomorphic sections of L and , respectively. Then Φ interpolates φ if and only if there exists a current w such that
.
In other words, φ has an interpolant if and only if there exist currents such that
, and
(14)
(14)
Moreover, in this case an interpolant of φ is given by
. Note that Φ is holomorphic since
. Here the last equality follows from (Equation9
(9)
(9) ).
Let us now consider interpolation on with respect to the line bundle
. Recall that there is a Hermitian resolution of
of the form (Equation1
(1)
(1) ). We write
for the
-valued component of
.
If for some current
, then one can successively find currents
such that (Equation14
(14)
(14) ) holds since, in view of (Equation10
(10)
(10) ),
and it follows from, e.g. [Citation10, Theorem 10.7] that
We thus have the following condition for the existence of an interpolant.
Lemma 3.2
A global holomorphic section φ of has an interpolant if and only if
is
-exact, i.e. there exists a current η such that
.
Proof
Proof of Theorem 1.1
By Serre duality we have that is
-exact if and only if
for all global
-closed
-valued
-forms η. Note that each such form is of the form
for some global holomorphic section h of
. Since
is
-exact if and only if each component
is, the statement follows from Lemma 3.2.
Proof
Proof of Corollary 1.2
Let , see (Equation3
(3)
(3) ), and let φ be a global holomorphic section of
. We have for each ℓ that
for all global holomorphic sections h of
. Indeed, if
, the only such h is the zero section, and if
, then
since
. Therefore φ has an interpolant by Theorem 1.1.
4. Polynomial interpolation
Let us now return to the topic of polynomial interpolation. Recall that the setting is that X is a complex subspace of such that
is a finite set of points. The aim of this section is to give some examples where we explicitly compute the residue current R associated with a Hermitian resolution of
and write down the moment conditions that Theorem 1.1 imposes on a function
for the existence of an interpolant of degree at most d. We do this by identifying g with a global holomorphic section φ of
and use the fact that g has an interpolant of degree at most d if and only if φ has an interpolant. More precisely, we let
denote homogeneous coordinates on
, and we view
as an open complex subspace of
via the embedding
. Recall that on
there is a frame e for
such that a global holomorphic section Φ of
is given by
where G is a polynomial of degree at most d on
.
Throughout this section we shall let ω in Theorem 1.1 be the nonvanishing holomorphic -valued n-form on
such that
on
.
Note that the dimension principle gives that , and throughout this section we write
rather than
for the
-valued component of
.
Example 4.1
Let . We have that X is defined by the homogeneous ideal
, where
and
. A Hermitian resolution of
is given by the Koszul complex, see Section 2.2, where we interpret
as a global holomorphic section of
. Thus the associated residue current takes values in
, and is given by the Coleff–Herrera product, see Section 2.2,
By a straightforward computation, cf.(Equation4
(4)
(4) ), we get
By Theorem 1.1, we now get the following. Since
for
, we have that any
has an interpolant of degree at most 2. Moreover, g has an interpolant of degree at most 1 if and only if (Equation2
(2)
(2) ) holds when
and h = 1. In view of (Equation5
(5)
(5) ) this amounts to
(15)
(15)
which is expected since the values of g at
,
, and
uniquely determines a polynomial of degree at most 1 that takes the value
at
. Note that this gives that the interpolation degree of X is 2.
We have that g has a constant interpolant if and only if (Equation2(2)
(2) ) holds for all global holomorphic h of
. By linearity we only need to check
, which amounts to (Equation15
(15)
(15) ),
, and
. This amounts to
as expected.
Example 4.2
Let . We have that X is defined by the homogeneous ideal
, where
and
. We have Hermitian resolutions of
and
and a map of complexes:
where the upper complex is the Koszul complex, see Section 2.2. Moreover,
and
Let R and
denote the residue currents associated with the resolutions of
and
, respectively. We have that
takes values in
and is given by the Coleff–Herrera product, see Section 2.2,
Thus by the comparison formula, see Section 2.3,
. A straightforward computation gives that
and
By Theorem 1.1, we now get the following. Since
for
, we have that any
has an interpolant of degree at most 2. Moreover, g has an interpolant of degree at most 1 if and only if (Equation2
(2)
(2) ) holds when
and h = 1. (Note that there is no condition involving
since
in this case.) In view of (Equation5
(5)
(5) ) we get the condition
(16)
(16)
which is expected since g has an interpolant of degree at most 1 if and only if
is the average of
and
. Note that this gives that the interpolation degree of X is 2.
We get that g has a constant interpolant if and only if (Equation2(2)
(2) ) holds when
for
, and when
and h = 1. Since
vanishes on the support of
, this amounts to Equation (Equation16
(16)
(16) ),
and
. This amounts to
as expected.
We end this note by considering Hermite interpolation. We refer to, e.g. [Citation11–13], and references therein for a classical survey of this topic.
Example 4.3
Let , and let g be a holomorphic function on the complex subspace
defined by the ideal generated by
. Here we allow for the possibility that
for some i, j, so that X is nonreduced in general, and we denote the number of times that
occurs by
. We have that a polynomial G interpolates g if and only if, for each
,
We say that a polynomial interpolates g with respect to
,
, if it interpolates the pull-back of g to the complex subspace defined by the ideal generated by
. We denote the unique polynomial that interpolates g with respect to
by
. The coefficient of its
-term is referred to as the kth divided difference of g and we denote it by
. By induction it is not difficult to see that
see [Citation11, Theorem 1.8]. This is referred to as Newton's formula.
We claim that has an interpolant of degree at most d if and only if
for all polynomials h of degree at most r−d−1. Let us show how this condition follows from Theorem 1.1. Since the ideal is generated by a single element, we have that the associated residue current is given by
Theorem 1.1 together with Stokes' formula gives that g has an interpolant if and only if
for all polynomials h of degree at most r−d−1, where
is a circle of radius
. Here we have used the fact that
interpolates gh. By letting
, a direct calculation gives that the second integral is equal to
, and hence the claim follows.
Acknowledgments
I would like to thank Elizabeth Wulcan and Mats Andersson for helpful discussions and comments on preliminary versions of this note.
Disclosure statement
No potential conflict of interest was reported by the author(s).
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