フレネ・セレの公式
曲線上の各点 r (s) において、3組のベクトル {T, N, B} を以下のように定義する:
T ≡ d r d s = r ′ ( t ) ‖ r ′ ( t ) ‖ ( 1 ) N ≡ d T / d s ‖ d T / d s ‖ = r ′ ( t ) × ( r ′′ ( t ) × r ′ ( t ) ) ‖ r ′ ( t ) × ( r ′′ ( t ) × r ′ ( t ) ) ‖ ( 2 ) B ≡ T × N = r ′ ( t ) × r ′′ ( t ) ‖ r ′ ( t ) × r ′′ ( t ) ‖ ( 3 ) {\displaystyle {\begin{aligned}{\boldsymbol {T}}&\equiv {\frac {\mathrm {d} {\boldsymbol {r}}}{\mathrm {d} s}}\\&={\frac {{\boldsymbol {r}}'(t)}{\left\|{\boldsymbol {r}}'(t)\right\|}}&(1)\\[1.