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\course{Elektrotechnik}
\module{FuVar}
\semester{Spring Semseter 2021}

\authoremail{naoki.pross@ost.ch}
\author{\textsl{Naoki Pross} -- \texttt{\theauthoremail}}

\title{\texttt{\themodule} Notes}
\date{\thesemester}

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\section*{License}
\doclicenseThis

\section{Scalar Fields}

\begin{definition}[Scalar Field]
  We call a function \(f\) a \emph{scalar field} when it maps values from
  \(\Rset^n \to \Rset\).
\end{definition}

\begin{definition}[Partial derivative of a scalar field]
  Let \(f: \Rset^n \to \Rset\), the \emph{partial} derivative of \(f\) with
  respect to \(x_k\), (\(0 < k < n\)), is defined as
  \[
    \frac{\partial f}{\partial x_k} :=
    \lim_{h \to 0} \frac{f(x_1, \dots, x_k + h, \dots, x_n) 
      - f(x_1, \dots, x_k, \dots, x_n)}{h}
    = \partial_{x_k} f(x, y)
  \]
  That is, we keep all variables of \(f\) fixed, except for \(x_k\).
\end{definition}

\begin{definition}[Tangent plane]
  For a scalar field \(f(x,y)\) we define the \emph{tangent plane} \(p(x,y)\)
  at coordinates \((x_0, y_0)\) to be:
  \[
    p(x, y) =
      f(x_0, y_0)
      + \partial_x f(x_0, y_0) (x - x_0)
      + \partial_y f(x_0, y_0) (y - y_0)
  \]
\end{definition}

The above can be used to calculate the one dimensional derivative of an implicit curve.

\begin{lemma}[Implicit derivative]
  The slope \(m\) of an implicit curve \(f(x,y)\) at the point \((x_0, y_0)\) is given by
  \[
    m = \partial_x f(x_0, y_0) / \partial_y f(x_0, y_0)
  \]
  of course only if \(\partial_y f(x_0, y_0) \neq 0\).
\end{lemma}

\begin{definition}[Total derivative]
  \[
    \dd{f}
  \]
\end{definition}

\end{document}