On Wed, 30 Dec 1998, J F Harper wrote:
> On Sat, 26 Dec 1998, Roger Cooke wrote:
>
>> I believe the classification of pde's as hyperbolic, elliptic, and
>> parabolic is due to IG Petrovsky (1901--1973).
>>
>
> When? It's in Hadamard's "Lectures on Cauchy's problem in linear partial
> differential equations" (written 1921 publ. 1923 Yale) with no indication
> it wasn't already well-known then.
>
> John Harper, School of Mathematical and Computing Sciences,
> Victoria University, Wellington, New Zealand
> e-mail john.harper@vuw.ac.nz phone (+64)(4)471 5341 fax (+64)(4)495 5045