Ca alors, c'est un gremlin

On the Geometry of an Integrable (2 + 1)-Dimensional Sine-Gordon ... - [
de WK Schief - 1997 -

Pik,l = PilPIk,. Pik,i + Pki,k + PliPlk. = 0. (2.6).
Since the vectors Xi form an orthonormal triad, they may be grouped into the orthogonal matrix ...www.jstor.org/stable/53013