Symmetry of second derivatives or Equality of mixed partials

Symmetry of second derivatives or Equality of mixed partials

https://en.wikipedia.org/wiki/Symmetry_of_second_derivatives#Schwarz's_theorem

https://en.wikipedia.org/wiki/Symmetry_of_second_derivatives

Смешанная частная производная

https://ru.wikipedia.org/wiki/%D0%A1%D0%BC%D0%B5%D1%88%D0%B0%D0%BD%D0%BD%D0%B0%D1%8F_%D1%87%D0%B0%D1%81%D1%82%D0%BD%D0%B0%D1%8F_%D0%BF%D1%80%D0%BE%D0%B8%D0%B7%D0%B2%D0%BE%D0%B4%D0%BD%D0%B0%D1%8F

Second

https://en.wikipedia.org/wiki/Second_derivative

https://livepcwiki.ru/wiki/Second_derivative