4 ( η 1 ( T 1 ) ⋅ η 2 ( T 2 ) - η 2 ( T 1 ) ⋅ η 1 ( T 2 ) ) = | T 1 ∩ T 2 | ( mod 2 ) 4(\boldsymbol{{\eta}}^{1}(T_{1})\cdot\boldsymbol{{\eta}}^{2}(T_{2})-\boldsymbol{{\eta}}^{2}(T_{1})\cdot\boldsymbol{{\eta}}^{1}(T_{2}))=|T_{1}\cap T_{2}|\pmod{2}