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Fig. 2.
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(a) The spiral density wave derived from the periodic orbits of the x1 family for the normal form Hamiltonian (29) of first order 
and (b) of second order 
, for Mb = 6.25 × 1010 M⊙ (a strong bar), Ωb = 45 km s−1 kpc−1, and Ωsp = 15 km s−1 kpc−1.
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