Shoshichi Kobayashi (小林 昭七, Kobayashi Shōshichi, 4 January 1932 – 29 August 2012)[1] was a Japanese mathematician. He was the eldest brother of electrical engineer and computer scientist Hisashi Kobayashi.[2] His research interests were in Riemannian and complexmanifolds, transformation groups of geometric structures, and Lie algebras.
Kobayashi served as chairman of the Berkeley Mathematics Department for a three-year term from 1978 to 1981 and for the 1992 Fall semester. He chose early retirement under the VERIP plan in 1994.
As a consequence of the Gauss–Codazzi equations and the commutation formulas for covariant derivatives, James Simons discovered a formula for the Laplacian of the second fundamental form of a submanifold of a Riemannian manifold.[4] As a consequence, one can find a formula for the Laplacian of the norm-squared of the second fundamental form. This "Simons formula" simplifies significantly when the mean curvature of the submanifold is zero and when the Riemannian manifold has constant curvature. In this setting, Shiing-Shen Chern, Manfredo do Carmo, and Kobayashi studied the algebraic structure of the zeroth-order terms, showing that they are nonnegative provided that the norm of the second fundamental form is sufficiently small.
As a consequence, the case in which the norm of the second fundamental form is constantly equal to the threshold value can be completely analyzed, the key being that all of the matrix inequalities used in controlling the zeroth-order terms become equalities. As such, in this setting, the second fundamental form is uniquely determined. As submanifolds of space forms are locally characterized by their first and second fundamental forms, this results in a complete characterization of minimal submanifolds of the round sphere whose second fundamental form is constant and equal to the threshold value. Chern, do Carmo, and Kobayashi's result was later improved by An-Min Li and Jimin Li, making use of the same methods.[5]
On a Kähler manifold, it is natural to consider the restriction of the sectional curvature to the two-dimensional planes which are holomorphic, i.e. which are invariant under the almost-complex structure. This is called the holomorphic sectional curvature. Samuel Goldberg and Kobayashi introduced an extension of this quantity, called the holomorphic bisectional curvature; its input is a pair of holomorphic two-dimensional planes. Goldberg and Kobayashi established the differential-geometric foundations of this object, carrying out many analogies with the sectional curvature. In particular they established, by the Bochner technique, that the second Betti number of a connected closed manifold must equal one if there is a Kähler metric whose holomorphic bisectional curvature is positive. Later, Kobayashi and Takushiro Ochiai proved some rigidity theorems for Kähler manifolds. In particular, if M is a closed Kähler manifold and there exists α in H1, 1(M, ℤ) such that
then M must be biholomorphic to complex projective space. This, in combination with the Goldberg–Kobayashi result, forms the final part of Yum-Tong Siu and Shing-Tung Yau's proof of the Frankel conjecture.[6] Kobayashi and Ochiai also characterized the situation of c1(M) = nα as M being biholomorphic to a quadratic hypersurface of complex projective space.
Kobayashi is also notable for having proved that a hermitian–Einstein metric on a holomorphic vector bundle over a compact Kähler manifold has deep algebro-geometric implications, as it implies semistability and decomposability as a direct sum of stable bundles.[7] This establishes one direction of the Kobayashi–Hitchin correspondence. Karen Uhlenbeck and Yau proved the converse result, following well-known partial results by Simon Donaldson.
In the 1960s, Kobayashi introduced what is now known as the Kobayashi metric. This associates a pseudo-metric to any complex manifold, in a holomorphically invariant way.[8] This sets up the important notion of Kobayashi hyperbolicity, which is defined by the condition that the Kobayashi metric is a genuine metric (and not only a pseudo-metric). With these notions, Kobayashi was able to establish a higher-dimensional version of the Alhfors–Schwarz lemma from complex analysis.
Kobayashi, Shoshichi (1972). Transformation groups in differential geometry. Ergebnisse der Mathematik und ihrer Grenzgebiete. Vol. 70. New York–Heidelberg: Springer-Verlag. ISBN0-387-05848-6. MR0355886. Zbl0246.53031.
