“…An almost D-complex structure is said integrable (and hence is called D-complex, or also para-complex ) if moreover T + X, T + X ⊆ T + X and T − X, T − X ⊆ T − X , (or, equivalently, if the Nijenhius tensor of K, defined by N K (·, ··) := [·, ··] + [K·, K · ·] − K [K·, ··] − K [·, K · ·] , vanishes). We refer, e.g., to [15,1,7,8,2,3,25,26] and the references therein for more results about (almost) D-complex structures and motivations for their study.…”