σ o c t = T i ( n ) n i = σ i j n i n j = σ 1 n 1 n 1 + σ 2 n 2 n 2 + σ 3 n 3 n 3 = 1 3 ( σ 1 + σ 2 + σ 3 ) = 1 3 I 1 {\displaystyle {\begin{aligned}\sigma _{\mathrm {oct} }&=T_{i}^{(n)}n_{i}\\&=\sigma _{ij}n_{i}n_{j}\\&=\sigma _{1}n_{1}n_{1}+\sigma _{2}n_{2}n_{2}+\sigma _{3}n_{3}n_{3}\\&={\tfrac {1}{3}}(\sigma _{1}+\sigma _{2}+\sigma _{3})={\tfrac {1}{3}}I_{1}\end{aligned}}}