“…For any given piezoelectricity tensor P ∈ Piez, an order two symmetry is define to be some 𝑔 = r(𝑛 𝑛 𝑛, 𝜋) or 𝑔 = s(𝑛 𝑛 𝑛) such that with 𝑛 𝑛 𝑛 = (𝑛 1 , 𝑛 2 , 𝑛 3 ) a unit vector in R 3 as unknown, leading to 18 algebraic equations of homogeneous degree 6 in (𝑛 1 , 𝑛 2 , 𝑛 3 ). In fact, following [45,44], it is possible to obtain reduced algebraic equations of degree 3.…”