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On variational approach to differential invariants of rank two distributions. (English) Zbl 1091.58002
Summary: We construct differential invariants for generic rank 2 vector distributions on $$n$$-dimensional manifolds, where $$n\geq 5$$. Our method for the construction of invariants is completely different from the Cartan reduction-prolongation procedure. It is based on the dynamics of the field of so-called abnormal extremals (singular curves) of rank 2 distribution and on the theory of unparameterized curves in the Lagrange Grassmannian, developed in [A. Agrachev, and I. Zelenko, J. Dyn. Control Syst. 8, No. 1, 93–140 (2002; Zbl 1019.53038); and ibid. 8, No. 2, 167–215 (2002; Zbl 1045.53051)]. In this way we construct the fundamental form and the projective Ricci curvature of rank 2 vector distributions for arbitrary $$n\geq 5$$.
In the next paper [I. Zelenko, Fundamental form and Cartan’s tensor of (2,5)-distributions coincide, J. Dyn. Control. Syst., in press, SISSA preprint, Ref. 13/2004/M, February 2004, math.DG/0402195] we show that in the case $$n=5$$ our fundamental form coincides with the Cartan covariant biquadratic binary form, constructed in 1910 in [É. Cartan, Ann. Sci. Ecole Norm. 27 (3), 109–192 (1910; JFM 41.0417.01); reprinted in: Œuvres complètes, Partie II, vol. 2, Gauthier-Villars, Paris (1953; Zbl 0058.08302)]. Therefore first our approach gives a new geometric explanation for the existence of the Cartan form in terms of an invariant degree four differential on an unparameterized curve in Lagrange Grassmannians. Secondly, our fundamental form provides a natural generalization of the Cartan form to the cases $$n > 5$$. Somewhat surprisingly, this generalization yields a rational function on the fibers of the appropriate vector bundle, as opposed to the polynomial function occurring when $$n=5$$. For $$n=5$$ we give an explicit method for computing our invariants and demonstrate the method on several examples.

##### MSC:
 58A30 Vector distributions (subbundles of the tangent bundles) 53A55 Differential invariants (local theory), geometric objects 58A17 Pfaffian systems
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##### References:
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