Article

    Ufa Mathematical Journal
    Volume 15, Number 1, pp. 56-121

    Inexistence of non-product Hessian rank 1 affinely homogeneous hypersurfaces HnRn+1 in dimension n


    Merker J.

    DOI:10.13108/2023-15-1-56

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    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.