Sections
A section (or cross section) of a fiber bundle is a continuous map f : B → E such that π(f(x))=x for all x in B. Since bundles do not in general have globally defined sections, one of the purposes of the theory is to account for their existence. The obstruction to the existence of a section can often be measured by a cohomology class, which leads to the theory of characteristic classes in algebraic topology.
The most well-known example is the hairy ball theorem, where the Euler class is the obstruction to the tangent bundle of the 2-sphere having a nowhere vanishing section.
Often one would like to define sections only locally (especially when global sections do not exist). A local section of a fiber bundle is a continuous map f : U → E where U is an open set in B and π(f(x))=x for all x in U. If (U, φ) is a local trivialization chart then local sections always exist over U. Such sections are in 1-1 correspondence with continuous maps U → F. Sections form a sheaf.
Read more about this topic: Fiber Bundle
Famous quotes containing the word sections:
“... many of the things which we deplore, the prevalence of tuberculosis, the mounting record of crime in certain sections of the country, are not due just to lack of education and to physical differences, but are due in great part to the basic fact of segregation which we have set up in this country and which warps and twists the lives not only of our Negro population, but sometimes of foreign born or even of religious groups.”
—Eleanor Roosevelt (18841962)
“Childhood lasts all through life. It returns to animate broad sections of adult life.... Poets will help us to find this living childhood within us, this permanent, durable immobile world.”
—Gaston Bachelard (18841962)
“That we can come here today and in the presence of thousands and tens of thousands of the survivors of the gallant army of Northern Virginia and their descendants, establish such an enduring monument by their hospitable welcome and acclaim, is conclusive proof of the uniting of the sections, and a universal confession that all that was done was well done, that the battle had to be fought, that the sections had to be tried, but that in the end, the result has inured to the common benefit of all.”
—William Howard Taft (18571930)