For a manifold M, we prove that any function defined on a vector bundle of basis M and quadratic at infinity has at least Qcat (M) + 1 critical points. Here Qcat (M) is a homotopically stable version of the LS-category defined by Scheerer, Stanley and Tanré [27]. The key homotopical result is that Qcat (M) can be identified with the relative LS-category of Fadell and Husseini [9] of the pair (M × D n+1 , M × S n ) for n big enough. Combining this result with the work of Laudenbach and Sikorav [19], we obtain that if M is closed, for any hamiltonian diffeomorphism with compact support ψ of T * M, #(ψ(M) ∩ M) ≥ Qcat (M) + 1, which improves all previously known homotopical estimates of this intersection number. (2000): 53D12, 55M30, 57R70.
Mathematics Subject Classification