Article
Ufa Mathematical Journal
Volume 15, Number 1, pp. 56-121
Inexistence of non-product Hessian rank 1
affinely homogeneous hypersurfaces Hn⊂Rn+1
in dimension n⩾
Merker J.
DOI:10.13108/2023-15-1-56
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Article on MathNetAbstact
Equivalences under the affine group
{\mathtt{Aff}}(\mathbb{R}^3) of constant Hessian rank 1 surfaces S^2 \subset \mathbb{R}^3,
sometimes called {\sl parabolic}, were, among other
objects, studied by
Doubrov, Komrakov, Rabinovich,
Eastwood, Ezhov, Olver, Chen, Merker, Arnaldsson, Valiquette.
In particular, homogeneous models and algebras of differential
invariants in various branches were fully understood.
{\sl Then what is about higher dimensions?} We consider hypersurfaces
H^n \subset \mathbb{R}^{n+1} graphed as \big\{ u = F(x_1, \dots, x_n)
\big\} whose Hessian matrix \big( F_{x_i x_j} \big),
a relative affine invariant, is similarly of constant rank 1.
{\sl Are there homogeneous models?}
Complete explorations were done by the author on a computer in
dimensions n = 2, 3, 4, 5, 6, 7. The first, expected outcome,
was a complete
classification of homogeneous models in dimensions
n = 2, 3, 4 (forthcoming article, case n = 2 already known).
The second, unexpected outcome, was that in dimensions n = 5, 6, 7,
there are no affinely homogenous models except those that are affinely equivalent to
a product of \mathbb{R}^m with a
homogeneous model in dimensions 2, 3, 4.
The present article establishes such a non-existence result
in every dimension n \geqslant 5,
based on the production of a normal form for
\big\{ u = F(x_1, \dots, x_n) \big\},
under {\mathtt{Aff}}(\mathbb{R}^{n+1}) up to order \leqslant n+5,
valid in any dimension n \geqslant 2.