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Total derivative
Function
| definition | $\dfrac{{\mathrm d}}{{\mathrm d}t}:\left(X_1\times\cdots \times X_n\times{\mathbb R}\to{\mathbb R}\right)\to \left(\left({\mathbb R}\to X_1\times\cdots \times X_n\right)\times {\mathbb R}\right)\to {\mathbb R}$ |
| let | $\diamond\ f(x^1,\dots,x^n,t)$ |
| definition | $\left(\dfrac{{\mathrm d}}{{\mathrm d}t}f\right)\left(t\mapsto\langle r^1(t),\dots,r^n(t)\rangle,t\right):=\sum_{j=1}^n \dfrac{\partial f}{\partial x^j}(\langle r^1(t),\dots,r^n(t),t\rangle)\cdot\dfrac{\partial r^j}{\partial t}(t)+\dfrac{\partial f}{\partial t}(\langle r^1(t),\dots,r^n(t),t\rangle)$ |
Example
The total derivative of the (not explicitly time dependent) function
$f:{\mathbb R}^3\to{\mathbb R}, \ \ \ f(x,y,t):=x^2\cos(y)$
along the trajectory
$R:{\mathbb R}\to{\mathbb R}^2, \ \ \ R:=t\mapsto\langle 7t,-3t^5\rangle$
is
$\left(\dfrac{{\mathrm d}}{{\mathrm d}t}f\right)(R,t)=2\,R_x(t)\cos(R_y(t))\cdot R_x'(t)+R_x(t)^2\sin(R_y(t))\cdot R_y'(t)$
$=2\cdot 7^2\,t\cos(-3t^5)-3\cdot 5\cdot 7^2\,t^{2+4}\sin(-3t^5)$
Reference
Wikipedia: Total derivative
Requirements
