In this paper, we consider horseshoe motion in the planar restricted threebody problem. On one hand, we deal with the families of horseshoe periodic orbits (HPOs) (which surround three equilibrium points called L 3 , L 4 and L 5 ), when the mass parameter µ is positive and small; we describe the structure of such families from the two-body problem (µ = 0). On the other hand, the region of existence of HPOs for any value of µ ∈ (0, 1/2] implies the understanding of the behaviour of the invariant manifolds of L 3 . So, a systematic analysis of such manifolds is carried out. As well the implications on the number of homoclinic connections to L 3 and on the simple infinite and double infinite period homoclinic phenomena are analysed. Finally, the relationship between the horseshoe homoclinic orbits and the HPO is considered in detail.
The spectra of the absorption coefficient (a) of GaSe a t the photon energy range 1.94 t o 2.64 eV and a t pressures between 1 bar and 12 kbar are investigated a t 295 O K . Data a t 1 t o 4360 bar confirm that the edge exciton of GaSe is the direct allowed one. The pressure coefficient of the band gap (dE,/dP = -6.2 x eV bar-I), one of the Rydberg exciton constant (dR/dP = -0.6 x eV bar-*), and one of the exciton effective massIt is supposed that two interband exciton transitions a t 2.001 and 3.25 eV give the contribution into the a spectrum above 2.1 eV. The assignment of these transitions is discussed in connection with the band structure calculation of Schluter. At 5280 bar a discontinuous jump of the absorption edge shape is observed for GaSe, with a change of t h e sign and a value for dE,/dP of +2.0 x 10-6 eV bar-' within the range of 5280 t o 11780 bar. This effect is interpreted as the transformation of one GaSe polytype into another, i.e. as a possible phase transformation close to t h a t of second kind. B HHTepBane 3~e p r~f i @oToHa OT i,94 no 2,64 eV y1 DaBneHHR OT 1 6ap no 12 kbar npH 295 O K accnenoBaHx cnempbI K O~@ @~U M~H T~ n o r n o q e~n~~ a ceneHana r a n n m GaSe. JJaHHbIe B ~n a n a a o~e OT 1 AO 4360 bar nonTsepmaamT, TO ~K C H T O H Ha KpaIo IIOrJIOIIJeHHFI GaSe RBJIReTCR IIpFIMbIM pa3peUIeHHbIM 3IECHTOHOM. npH6JIPiweHHO OIrpe~eneHbI K03@@Pi4HeHTbI HaBneHHR IUllpllHbI a a n p e q e~~o a 30HbI (dEg/dP = ---6,2 x eV -bar-I), ~C
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