MSC: 34B15 34B24 46T05 Keywords: Sturm-Liouville Monodromy Floquet matrix Infinite-dimensional manifolds with singularitiesWe study the critical set C of the nonlinear differential operatorxy ⊂ R 3 be the plane z = 0 and, for n > 0, let n be the cone x 2 + y 2 = tan 2 z, |z − 2πn| < π /2; also set Σ = R 2 xy ∪ n>0 n . For a generic smooth nonlinearity f : R → R with surjective derivative, we show that there is a diffeomorphism between the pairs (H p (S 1 ), C) and (R 3 , Σ) × H where H is a real separable infinite-dimensional Hilbert space.