Differential on a Riemann surface
A differential form on a Riemann surface that is invariant with respect to a conformal transformation of the local uniformizing parameter
. Differentials of the first order are most often encountered; these are differential forms of dimension 1 that are linear with respect to the differential of each of the variables
, of the form
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and that are invariant with respect to parameter change with sufficiently smooth coefficients . Differentials of order zero are sufficiently smooth complex functions
,
that are invariant with respect to parameter change, i.e. functions of the points
; differentials of the second order have the form
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All differentials of order on a Riemann surface vanish identically.
The addition of differentials of the same order on a Riemann surface is effected in the usual way:
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and is commutative and associative. The exterior multiplication of differentials on a Riemann surface is distributive with respect to addition, is denoted by the symbol and is defined by the following laws:
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Hence
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In general, exterior multiplication of a differential of order by a differential of order
, where
, yields a differential or order
, while if
it vanishes. The linear differentiation operator
transforms a differential of order
to a differential of order
:
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Moreover,
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and . Of importance to differentials on a Riemann surface is also the linear star conjugation operator
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Here
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The star conjugation operator is not identical with the complex conjugation operator. The latter is denoted by a bar: if , then
,
,
; also
,
. The Laplace operator
is defined on differentials of order zero:
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A differential is called exact if there exists a function
on
such that
everywhere; if
,
is called co-exact; if
on
,
is called closed; and if
,
is called a co-closed differential. Exactness entails closedness, but the opposite is not true. Let
be cycles on
. The integrals
known as the periods of the differential
, are defined in the usual way with the aid of a local uniformizing parameter. If
and
are homologous on
and
is a closed differential, then
, i.e. the periods of a closed differential depend only on the homology classes. All periods of an exact differential are equal to zero. Conversely, a closed differential is exact if and only if all its periods are equal to zero.
A function is said to be harmonic on
if
. A differential
is said to be a harmonic differential on
if
is closed and co-closed:
. A harmonic differential
is the total differential of a harmonic function in a neighbourhood of each point of
. If two real-valued functions
on
are connected by the relation
, they are conjugate harmonic functions satisfying the Cauchy–Riemann equations. Consequently, a function
is regular analytic, or holomorphic, on
if
. A differential
is said to be a regular analytic, or holomorphic, differential on
if
and
. A holomorphic differential
is the total differential of a holomorphic function in a neighbourhood of each point of
. A holomorphic differential
can be locally represented as
, where
and
is a holomorphic function in
.
The equivalence classes of measurable complex differentials on a Riemann surface for which the integral is finite form a Hilbert space
with the usual addition, multiplication by complex scalars and scalar product
. Each differential
from the class
is uniquely representable as the sum
where
and
is a harmonic differential on the Riemann surface.
The above harmonic and holomorphic functions or differentials of class on
are said to be regular on
. Let a differential
be defined, for example in a deleted neighbourhood
of a point
, which is harmonic in
. One then says that the harmonic differential
has singularity
in
if the difference
is a regular harmonic differential.
Similar definitions are utilized for harmonic and analytic functions, analytic differentials, etc. In particular, in the case of an analytic differential it is usually assumed that the function
is either regular analytic in a neighbourhood of each point
or else has only isolated singular points on
of single-valued character. An analytic differential
on
which has only singularities of pole type,
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is called a meromorphic differential; here and
is the order of the pole; if
, the pole is called simple:
is called the residue of the differential
at the pole
. A meromorphic differential on a compact Riemann surface
is also called an Abelian differential. A harmonic function on
or on some domain
with given singularities is also known as an Abelian potential.
The integration of Abelian differentials yields Abelian integrals (cf. Abelian integral), which actually account for all integrals of algebraic functions. In studying analytic differentials on an arbitrary, usually non-compact, Riemann surface , the natural requirement of preservation of the basic features of the classical theory of differentials on compact Riemann surfaces results in imposing additional conformally invariant restrictions on the regular differentials under study. The restriction most often used is the condition of integrability of, for example, the square of an analytic differential
, i.e. the Dirichlet integral
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must be finite.
Of fundamental importance in the theory of differentials on Riemann surfaces is the problem of the existence of a harmonic and analytic differential with given singularities on an arbitrary Riemann surface . This problem is directly connected with the global uniformization problem for Riemann surfaces, since the construction of a global uniformizing parameter requires the ability to construct a differential with given singularities.
The principal results concerning the existence problem are given below.
If a cycle not homologous to zero exists on
, there also exists on
an everywhere-regular harmonic differential
with period
and an everywhere-regular differential
. These differentials are not exact, and for this reason their integration does not yield single-valued harmonic or analytic functions on
. On a compact Riemann surface all harmonic and exact differentials are identically equal to zero. On the contrary, on a non-compact Riemann surface there exist everywhere-regular exact harmonic and holomorphic differentials which are not identically equal to zero.
Let be a fixed point of an arbitrary Riemann surface
and let
be an arbitrary natural number. Then there exist: an exact harmonic differential with singularity
at
; an exact real harmonic differential with singularity
(or
) at
; a harmonic function with singularity
at
; and an analytic differential with singularity
at
whose real part is an exact differential.
Let and
be distinct points of
. Then there exist: a harmonic or an analytic differential on
with singularities
at
and
at
; and a real harmonic function on
with singularities
at
and
at
.
Let be any pairwise distinct points on
and let
be arbitrary non-zero complex numbers such that
. There exists a harmonic or an analytic differential on
which is everywhere regular except at
, while at the points
it has simple poles with corresponding residues
,
.
The solution of the Dirichlet problem is also possible for Jordan domains , i.e. domains whose boundary
consists of
non-intersecting Jordan curves
.
The theory of differentials is the most advanced for compact Riemann surfaces . Let the genus (cf. Genus of a surface) of
be
. The vector space
of regular harmonic differentials
over the field of complex numbers has dimension
. If
are the cycles of a canonical basis of the homology of
, one may choose as a canonical basis of the differentials in
the differentials
,
, with period one along
and with period zero along
,
, and along all
; furthermore,
,
, have period one along
and period zero along
,
, and along all
. Any harmonic differential
can be represented in the form of a linear combination
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where are the so-called
-periods of
along the cycles
, and
are the
-periods of
along the cycles
.
Holomorphic differentials on a compact Riemann surface are called Abelian differentials of the first kind. The dimension of the vector space of holomorphic differentials is . All the differentials on a Riemann surface which have just been discussed can be expressed in terms of the variables
and
, e.g.
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etc. As distinct from complex manifolds of higher dimension, on Riemann surfaces only the exterior differential forms of types ,
,
, and
, having respective form
,
,
,
, are non-trivial. An analytic differential depends only on
:
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Non-linear differential forms of the type , where
and
are integers, are also employed. They are also known as differentials of type
or of dimension
. Differentials of type
are known as functions, those of type
as linear differentials, those of type
as inverse differentials, and those of type
as quadratic differentials. Quadratic differentials are the most frequently used. See also Global structure of trajectories; Local structure of trajectories; Quadratic differential; Riemann surface; Uniformization.
References
[1] | G. Springer, "Introduction to Riemann surfaces" , Chelsea, reprint (1981) |
[2] | R. Nevanlinna, "Uniformisierung" , Springer (1953) |
[3] | M. Schiffer, D.C. Spencer, "Functionals of finite Riemann surfaces" , Princeton Univ. Press (1954) |
[4] | H. Behnke, F. Sommer, "Theorie der analytischen Funktionen einer komplexen Veränderlichen" , Springer (1955) |
Comments
Differentials on a Riemann surface are simply complex-valued differential forms on the underlying -dimensional manifold (cf. also Differential form). Their local expressions in terms of local coordinates, however, are (usually) only written down with respect to local coordinates
such that
is a local uniformizing parameter (i.e.
is a local coordinate on the complex manifold). This is the meaning of the phrase "invariant with respect to conformal transformations of the local uniformizing parameter" and other phrases with "invariant" in them in the article above. The definition of the
-operator only makes sense for local expressions
for a differential form
such that
is a local uniformizing parameter (because the conformal coordinate changes satisfy the Cauchy–Riemann equations). The differential
is also called the conjugate differential of
.
A pure differential is one for which . This means it can be written as
(for some (not necessarily holomorphic) function
).
References
[a1] | H.M. Farkas, I. Kra, "Riemann surfaces" , Springer (1980) |
[a2] | L.V. Ahlfors, L. Sario, "Riemann surfaces" , Princeton Univ. Press (1960) pp. Chapt. V |
Differential on a Riemann surface. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Differential_on_a_Riemann_surface&oldid=24419