L ( q _ , q ˙ _ , t ) := m 2 q ˙ 2 − e ( q ˙ _ . A _ ( q _ , t ) − ϕ ( q _ , t ) ) {\displaystyle L\left({\underline {q}},{\underline {\dot {q}}},t\right):={\frac {m}{2}}{{\dot {q}}^{2}}-e\left({\underline {\dot {q}}}.{\underline {A}}\left({\underline {q}},t\right)-\phi \left({\underline {q}},t\right)\right)}