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First Cousin gluing on the pseudoconvex domain C

Statement

Assume the Axiom of Choice (AC). On C the sets U1:={z:∣z∣<2},U2:={z:∣z∣>1} and the functions m1:=1/z on U1 and m2:=0 on U2 form compatible first-Cousin data on the Hartogs pseudoconvex domain C in the sense of First Cousin problem on a pseudoconvex domain: the cover is finite, each mi is meromorphic on Ui, and m1−m2=1/z is holomorphic on U1∩U2={1<∣z∣<2}. The global meromorphic function G:=1/z satisfies G−m1=0∈O(U1) and G−m2=1/z∈O(U2), so it realizes the prescribed simple pole at 0.

Facts & Assumptions

Given: The Axiom of Choice; the plane C; the sets U1, U2; the functions m1=1/z on U1 and m2=0 on U2; the candidate G=1/z.

[F1]

(First Cousin problem on a pseudoconvex domain.) If Ω is a Hartogs pseudoconvex domain, (Ui)i∈I a locally finite open cover of Ω and mi meromorphic on Ui with mi−mj holomorphic on Ui∩Uj for all i,j, then there is a meromorphic G on Ω with G−mi holomorphic on Ui for every i.

[F2]

A meromorphic function on an open U is a function on an open dense D⊆U, holomorphic there, which near each point of U equals a quotient f/g of holomorphic functions with g not identically zero on any component; every holomorphic function is meromorphic, and "F−G is holomorphic on V" means that the difference admits a holomorphic extension to V (Meromorphic functions on an open set in complex Euclidean space).

[F3]

When Ω=Cm one has δΩ≡+∞, the boundary function is by convention the constant function 0, and the whole space is Hartogs pseudoconvex (Plurisubharmonic exhaustions and Hartogs pseudoconvexity).

[F4]

If g:U→C is complex differentiable at a with g(a)≠0, then 1/g is complex differentiable at a with (1/g)′(a)=−g′(a)/g(a)2, and the identity function has derivative 1 (Linearity, product, reciprocal, and quotient rules for complex derivatives); a function is holomorphic on U when it is complex differentiable at every point of U (Holomorphic functions on an open subset of Cm).

[F5]

The Euclidean ball B(0,2)⊆C is convex and hence path-connected, and path-connected sets are connected (Every convex subset of Rn, in particular every ball and Rn itself, is path-connected and hence connected, Every path-connected space is connected, and every path component lies inside a component), while the exterior {z:∣z∣>1} is open and path-connected (The exterior of a closed disc in the plane is path-connected); balls are open and closed balls are closed in a metric space (Arbitrary unions and finite intersections of open sets are open, open balls are open and closed balls are closed).

[F6]

AC is the assertion that every family of nonempty sets has a choice function (The Axiom of Choice).

Choice use. AC is the ambient hypothesis recorded in the Statement and cited as [F6]; it is consumed only inside the supplier theorem [F1], whose proof carries its own choice hypotheses. The example exhibits G by an explicit formula and selects nothing.

Proof

technique · direct
1.1F5givenalgebra

(The cover.) By [F5] the set U1=B(0,2) is open, convex, path-connected and connected, and U2={z:∣z∣>1}=C∖B‾(0,1) is open and path-connected, hence connected; moreover U1∪U2=C and U1∩U2={z:1<∣z∣<2}, so (U1,U2) is a finite, hence locally finite, open cover of C by domains.

1.2F2F4givenalgebra

(The meromorphic data.) By [F4] the identity z↦z is holomorphic on C and z↦1/z is holomorphic on C∖{0}; hence m1=1/z is meromorphic on U1 in the sense of [F2]: its domain U1∖{0} is open and dense in U1, m1 is holomorphic there, and at every point of U1 it equals f/g with f:=1 and g:=z, the denominator g not vanishing identically on any component of U1. Likewise m2=0 is holomorphic, hence meromorphic, on U2.

2.1F1F2F3F4step 1.2algebra

(Compatibility.) On the overlap U1∩U2={1<∣z∣<2}, which does not contain 0, the difference m1−m2=1/z is holomorphic by [F4]; this is the compatibility clause of [F1], and C is Hartogs pseudoconvex by [F3], so the data satisfy the hypotheses of [F1].

3.1F1step 2.1

(Existence by the Cousin theorem.) By [F1] there is a meromorphic function G on C with G−mi holomorphic on Ui for i=1,2.

4.1F1F2F4step 1.2step 3.1algebra

(The explicit solution.) The function G=1/z is meromorphic on C with domain C∖{0} by [F2] and [F4]; moreover G−m1=0 is holomorphic on U1, and G−m2=1/z is holomorphic on U2 because 0∉U2 and z↦1/z is holomorphic on C∖{0} by [F4]. So G=1/z is a solution in the sense of [F1]: it differs from mi by a holomorphic function on each Ui, and its only pole is the simple pole at 0 with principal part 1/z, which is exactly the pole prescribed by the data.

5.1F6step 4.1∎

(Conclusion.) The sets U1,U2 and the functions m1,m2 are compatible first-Cousin data on the Hartogs pseudoconvex domain C, and the global meromorphic function G=1/z realizes the prescribed simple pole at the origin, as claimed under the ambient Axiom of Choice [F6].

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