# Schoen–Yau conjecture

In mathematics, the Schoen–Yau conjecture is a disproved conjecture in hyperbolic geometry, named after the mathematicians Richard Schoen and Shing-Tung Yau.

It was inspired by a theorem of Erhard Heinz (1952). One method of disproof is the use of Scherk surfaces, as used by Harold Rosenberg and Pascal Collin (2006).

## Setting and statement of the conjecture

Let ${\displaystyle \mathbb {C} }$ be the complex plane considered as a Riemannian manifold with its usual (flat) Riemannian metric. Let ${\displaystyle \mathbb {H} }$ denote the hyperbolic plane, i.e. the unit disc

${\displaystyle \mathbb {H} :=\{(x,y)\in \mathbb {R} ^{2}|x^{2}+y^{2}<1\}}$

endowed with the hyperbolic metric

${\displaystyle \mathrm {d} s^{2}=4{\frac {\mathrm {d} x^{2}+\mathrm {d} y^{2}}{(1-(x^{2}+y^{2}))^{2}}}.}$

E. Heinz proved in 1952 that there can exist no harmonic diffeomorphism

${\displaystyle f:\mathbb {H} \to \mathbb {C} .\,}$

In light of this theorem, Schoen conjectured that there exists no harmonic diffeomorphism

${\displaystyle g:\mathbb {C} \to \mathbb {H} .\,}$

(It is not clear how Yau's name became associated with the conjecture: in unpublished correspondence with Harold Rosenberg, both Schoen and Yau identify Schoen as having postulated the conjecture). The Schoen(-Yau) conjecture has since been disproved.

The emphasis is on the existence or non-existence of an harmonic diffeomorphism, and that this property is a "one-way" property. In more detail: suppose that we consider two Riemannian manifolds M and N (with their respective metrics), and write

${\displaystyle M\sim N\,}$

if there exists a diffeomorphism from M onto N (in the usual terminology, M and N are diffeomorphic). Write

${\displaystyle M\propto N}$

if there exists an harmonic diffeomorphism from M onto N. It is not difficult to show that ${\displaystyle \sim }$ (being diffeomorphic) is an equivalence relation on the objects of the category of Riemannian manifolds. In particular, ${\displaystyle \sim }$ is a symmetric relation:

${\displaystyle M\sim N\iff N\sim M.}$

It can be shown that the hyperbolic plane and (flat) complex plane are indeed diffeomorphic:

${\displaystyle \mathbb {H} \sim \mathbb {C} ,}$

so the question is whether or not they are "harmonically diffeomorphic". However, as the truth of Heinz's theorem and the falsity of the Schoen–Yau conjecture demonstrate, ${\displaystyle \propto }$ is not a symmetric relation:

${\displaystyle \mathbb {C} \propto \mathbb {H} {\text{ but }}\mathbb {H} \not \propto \mathbb {C} .}$

Thus, being "harmonically diffeomorphic" is a much stronger property than simply being diffeomorphic, and can be a "one-way" relation.

## References

• Heinz, Erhard (1952). "Über die Lösungen der Minimalflächengleichung". Nachr. Akad. Wiss. Göttingen. Math.-Phys. Kl. Math.-Phys.-Chem. Abt. 1952: 51–56.
• Collin, Pascal; Rosenberg, Harold (2010). "Construction of harmonic diffeomorphisms and minimal graphs". Ann. of Math. 2. 172 (3): 1879–1906. arXiv:math/0701547. doi:10.4007/annals.2010.172.1879. ISSN 0003-486X. MR2726102