Besides, the torsion is

with τ̄Quad =

Proof. We carry out the same computation as in the proof of Lemma 10.3, now up to the order 2 in the r̃js. The truncated expression is a trigonometric polynomial in the angles φ̃ j of degree 4. Eliminating nonresonant monomials, that is, functions of k φ with k αQuad(0), is a classical matter. Two kinds of terms cannot be eliminated by averaging:

Monomials in the angle 4φ̃ 4.

Monomials in φ̃ 1

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