## Ragnar Sigurðsson, University of Iceland

Math colloquium

**Speaker**: Ragnar
Sigurðsson, University of Iceland

**Title: **Norms on complexifications of real vector
spaces.

Room: VRII-258

Time: Thursday February 6^{th}, 10:50 hrs.

#### Abstract:

The subject of this lecture is of general interest and it only requires knowledge of elementary linear algebra.

The complexification V_C of a real vector space

V is the smallest complex vector space which contains V

as a real subspace. If V is a normed space, then it is

of interest to know how norms may extend from V to V_C.

I will look at a real normed space V and give formulas

for the smallest and largest extension of a general norm

on V to a norm on V_C. These formulas are not explicit

so it is of interest to find explicit formulas in particular

examples. This is possible for extentions of norms induced

by inner products. The Lie norm is the largest

extension of the Euclidean norm on R^n to a complex norm

on C^n.

In complex analysis we deal a lot with plurisubharmonic

functions and an important source for examples are

functions of the form log||f||, where f is a holomorphic

map from a complex manifold into C^n and ||.|| is a norm

on C^n. In his thesis, Auðunn Skúta Snæbjarnarson, studied

the Lie norm on C^n and calculated interesting formulas for

the so called Monge-Ampere measure of log||f||, which is

indeed not an easy task.