“…Let (x(t, p 0 ), p(t, p 0 )) = γ(t, p 0 ) for (t, p 0 ) ∈ [0, t f ] × P and assume that the final time is fixed; a fold singularity occurs at a time t c ∈ (0,t f ] if det [∂x(t c ,p 0 )/∂p 0 ] = 0 [22]. Hence, conjugate points for fixed-time orbital transfer problems are tested by detecting the zero of det [∂x(·,p 0 )/∂p 0 ] on (0,t f ] in [23,24,26]. However, according to Eq.…”