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 Tools annual

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 Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

This paper contains a classification scheme and normal form results for fully non-linear C∞ systems of two first order p.d.e.s for two functions of two variables. The approach, based on E. Cartan’s theory of exterior differential systems, exploits, in the parabolic and hyperbolic cases, the invariant geometrical properties of the characteristic and focal systems. An existence theorem is also given for the initial value problem for C∞ parabolic systems with completely integrable characteristics and non-focal Cauchy data.

Normal forms and focal systems for determined systems of two first-order partial differential equations