Bloch's principle
Bloch's principle is a philosophical principle in mathematics
stated by André Bloch.{{cite news|first=A.|last=Bloch|
title=La conception actuelle de la theorie de fonctions entieres et meromorphes|journal=Enseignement Math.|year=1926| volume=25|pages=83–103}}
Bloch states the principle in Latin as: Nihil est in infinito quod non prius fuerit in finito, and explains this as follows: Every proposition in whose statement the actual infinity occurs can be always considered a consequence, almost immediate, of a proposition where it does not occur, a proposition in finite terms.
Bloch mainly applied this principle to the theory of functions of a complex variable. Thus, for example, according to this principle, Picard's theorem corresponds to Schottky's theorem, and Valiron's theorem corresponds to Bloch's theorem.
Based on his Principle, Bloch was able to predict or conjecture several
important results such as the Ahlfors's Five Islands theorem,
Cartan's theorem on holomorphic curves omitting hyperplanes,{{cite book|ref=la|first=S.|last=Lang|
title=Introduction to complex hyperbolic spaces|publisher=Springer Verlag|year=1987}} Hayman's result that an exceptional set of radii is unavoidable in Nevanlinna theory.
In the more recent times several general theorems were proved which can be regarded as rigorous statements in the spirit of the Bloch Principle:
Zalcman's lemma
A family of functions meromorphic on the unit disc is not normal if and only if there exist:
- a number
- points
- functions
- numbers
such that
spherically uniformly on compact subsets of where is a nonconstant meromorphic function on {{cite journal|first=L.|last=Zalcman|
title=Heuristic principle in complex function theory|journal=Amer. Math. Monthly|
volume=82|year=1975|issue=8|pages=813–817|doi=10.1080/00029890.1975.11993942}}
Zalcman's lemma may be generalized to several complex variables. First, define the following:
A family of holomorphic functions on a domain is normal in if every sequence of functions contains either a subsequence which converges to a limit function uniformly on each compact subset of or a subsequence which converges uniformly to on each compact subset.
For every function of class define at each point a Hermitian form
and call it the Levi form of the function at
If function is holomorphic on set
This quantity is well defined since the Levi form is nonnegative for all
In particular, for the above formula takes the form
and coincides with the spherical metric on
The following characterization of normality can be made based on Marty's theorem, which states that a family is normal if and only if the spherical derivatives are locally bounded:{{cite book
| author = P. V. Dovbush (2020)
| title = Zalcman's lemma in Cn, Complex Variables and Elliptic Equations, 65:5, 796-800, DOI: 10.1080/17476933.2019.1627529
| doi = 10.1080/17476933.2019.1627529
| s2cid = 198444355
}}
Suppose that the family of functions holomorphic on is not normal at some point Then there exist sequences such that the sequence converges locally uniformly in to a non-constant entire function satisfying
Brody's lemma
Let X be a compact complex analytic manifold, such that every holomorphic map from the complex plane
to X is constant. Then there exists a metric on X such that every holomorphic map from the unit disc with the Poincaré metric to X does not increase distances.Lang (1987).