English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

In his 1954 paper about the initial value problem for 2D hyperbolic nonlinearPDEs, P. Lax declared that he had "a strong reason to believe" that there mustexist a well-defined class of "not genuinely nonlinear" nonlinear PDEs. In 1978G. Boillat coined the term "completely exceptional" to denote it. In the caseof $2^{\textrm{nd}}$ order (nonlinear) PDEs, he also proved that this classreduces to the class of Monge-Amp\`ere equations. We review here, against aunified geometric background, the notion of complete exceptionality, thedefinition of a Monge-Amp\`ere equation, and the interesting link between them.

An introduction to completely exceptional $2^{\textrm{nd}}$ order scalar PDEs