Finite Elemente – B-Splines | Vorlesungsmitschriebe (2024)

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Das Spline-Konzept

Polynome stellen zwar gute lokale Approximationen für glatte Funktionen dar, allerdings kann die Genauigkeit auf großen Intervallen sehr klein sein. Außerdem haben lokale Änderungen einen globalen Einfluss.Daher ist der Übergang zu stückweise Polynomen sozusagen „natürlich“.

Spline: Ein Spline vom Grad \(\le n\) mit Gitterweite \(h\) ist \((n - 1)\)-fach stetig differenzierbar und stimmt auf jedem Gitterintervall \([i, i+1]h\) desParameterintervalls \(D\) mit einem Polynom vom Grad \(\le n\) überein.

Diese Definition eignet sich natürlich nicht für numerische Berechnungen, daher muss eine lokale Basis konstruiert werden. Die Basis, die hier verwendet wird, kann durch den linearen Fall (Hut-Funktionen)motiviert werden.

Definition und grundlegende Eigenschaften

B-Spline: Der uniforme B-Spline vom Grad \(n\) ist definiert durch die Rekursion

\(\seteqnumber{0}{}{0}\)

\begin{align*}b^n(x) := \int _{x-1}^x b^{n-1},\end{align*}beginnend mit der charakteristischen Funktion \(b^0\) des Einheitsintervalls \([0, 1)\). Äquivalent ist die Rekursion

\(\seteqnumber{0}{}{0}\)

\begin{align*}\frac {d}{dx} b^n(x) := b^{n-1}(x) - b^{n-1}(x - 1)\end{align*}mit \(b^n(0) = 0\).

Eigenschaften von B-Splines: B-Splines erfüllen die folgenden Eigenschaften:

  • Positivität und lokaler Träger: \(b^n\) ist positiv auf \((0, n + 1)\) und verschwindet außerhalb dieses Intervalls (außer für \(n = 0\), hier gilt \(b^0(0) = 1\)).

  • Glattheit: \(b^n\) ist \((n - 1)\)-fach stetig differenzierbar, wobei die \(n\)-te Ableitung in den Knotenpunkten \(0, \dotsc , n + 1\) unstetig ist.

  • Struktur als stückweises Polynom: \(b^n\) ist auf jedem Intervall \([k, k + 1)\), \(k = 0, \dotsc , n\), ein Polynom vom Grad \(n\).

Symmetrie und Monotonie: Der B-Spline vom Grad \(n\) ist symmetrisch, d. h.

\(\seteqnumber{0}{}{0}\)

\begin{align*}b^n(x) = b^n(n + 1 - x),\end{align*}und auf \([0, (n + 1)/2]\) und \([(n + 1)/2, n + 1]\) strikt monoton.

Rekursionsformel

Rekursionsformel: Der B-Spline \(b^n\) ist eine gewichtete Summe von B-Splines vom Grad \(n - 1\):

\(\seteqnumber{0}{}{0}\)

\begin{align*}b^n(x) = \frac {x}{n} b^{n-1}(x) + \frac {n + 1 - x}{n} b^{n-1}(x - 1).\end{align*}

Taylor-Koeffizienten: Die \(n + 1\) polynomialen Segmente

\(\seteqnumber{0}{}{0}\)

\begin{align*}a_{k,0}^n + a_{k,1}^n t + \dotsb + a_{k,n}^n t^n,\quad t = x - k \in [0, 1),\end{align*}des B-Splines \(b^n\) können mit der Rekursion

\(\seteqnumber{0}{}{0}\)

\begin{align*}a_{k,\ell }^n = \frac {k}{n} a_{k,\ell }^{n-1} + \frac {1}{n} a_{k,\ell -1}^{n-1} + \frac {n + 1 - k}{n} a_{k-1,\ell }^{n-1} - \frac {1}{n} a_{k-1,\ell -1}^{n-1}\end{align*}berechnet werden, wobei \(a_{0,0}^1 := 1\) und \(a_{k,\ell }^n := 0\) für \(k \notin \{0, \dotsc , n\}\) oder \(\ell \notin \{0, \dotsc , n\}\).

Darstellung von Polynomen

kardinale Splines: Für \(h > 0\) und \(k \in \integer \) sind

\(\seteqnumber{0}{}{0}\)

\begin{align*}b_{k,h}^n(x) := b^n(x/h - k)\end{align*}B-Splines auf dem Gitter \(h\integer \). Ihre Linearkombinationen \(\sum _{k \in \integer } c_k b_{k,h}^n\) heißen kardinaleSplines vom Grad \(\le n\) mit Gitterweite \(h\).

Marsden-Identität: Für \(x, t \in \real \) gilt

\(\seteqnumber{0}{}{0}\)

\begin{align*}(x - t)^n = \sum _{k \in \integer } \psi _{k,h}^n(t) b_{k,h}^n(x),\end{align*}wobei \(\psi _{k,h}^n(t) := h^n (k + 1 - t/h) \dotsm (k + n - t/h)\).

lineare Unabhängigkeit: Für jedes Gitterintervall \([\ell , \ell + 1)h\) sind die B-Splines \(b_{k,h}\),
\(k = \ell - n, \dotsc , \ell \), die auf diesem Intervall nicht verschwinden, linear unabhängig.

Subdivision

Gitterverfeinerung: Der B-Spline \(b_{k,h}^n\) kann als Linearkombination von B-Splines mit Gitterweite \(h/2\) geschrieben werden:

\(\seteqnumber{0}{}{0}\)

\begin{align*}b_{k,h}^n = 2^{-n} \sum _{\ell =0}^{n+1} \binom {n+1}{\ell } b_{2k+\ell ,h/2}^n.\end{align*}

Subdivisionsalgorithmus: Die Koeffizienten \(c_\ell ’\) eines kardinalen Splines \(\sum _k c_k b_{k,h}^n\) bzgl. der halben Gitterweite \(h/2\) können wie folgt berechnet werden:

  • Zunächst setzt man \(c_{2k}’ := c_{2k+1}’ := c_k\).

  • Anschließend bildet man simultan Mittelwerte, d. h. \(c_\ell ’ \leftarrow \frac {1}{2} (c_\ell ’ + c_{\ell -1}’)\), \(\ell \in \integer \).
    Dieser Schritt wird \(n\)-mal insgesamt wiederholt.

Skalarprodukte

Faltung: Die Faltung zweier B-Splines ist ein B-Spline höheren Grades: \(\seteqnumber{0}{}{0}\)

\begin{align*}b^{m+n+1}(x) = \int _\real b^m(x - y) b^n(y) \dy .\end{align*}

Skalarprodukte: Die Skalarprodukte der B-Splines \(b_{k,h}^n\) und \(b_{\ell ,h}^n\) und ihrer Ableitungen sind

\(\seteqnumber{0}{}{0}\)

\begin{align*}s_{k-\ell }^n &:= h b^{2n+1} (n + 1 + k - \ell ),\\ d_{k-\ell }^n &:= h^{-2} (2s_{k-\ell }^{n-1} - s_{k-\ell -1}^{n-1} - s_{k-\ell +1}^{n-1}).\end{align*}

Tabelle: Skalarprodukte der B-Splines und ihrer Ableitungen für \(h = 1\):

\(\seteqnumber{0}{}{0}\)

\begin{align*}\begin{array}{c||cccc|cccc} n & s_0^n & s_1^n & s_2^n & s_3^n & d_0^n & d_1^n & d_2^n & d_3^n\\\hline 1 & \frac {2}{3} & \frac {1}{6} & & &2 & -1 & &\\ 2 & \frac {11}{20} & \frac {13}{60} & \frac {1}{120} & & 1 & -\frac {1}{3} & -\frac {1}{6} &\\ 3 & \frac {151}{315} & \frac{397}{1680} & \frac {1}{42} & \frac {1}{5040} & \frac {2}{3} & -\frac {1}{8} & -\frac {1}{5} & -\frac {1}{120} \end {array}\end{align*}Wegen Symmetrie gilt \(s_i^n = s_{-i}^n\) und \(d_i^n = d_{-i}^n\).

Skalarprodukte von Ableitungen höherer Ordnung: Skalarprodukte mit höheren Ableitungen können durch die Differentiationsformel

\(\seteqnumber{0}{}{0}\)

\begin{align*}\frac {d^\alpha }{dx^\alpha } b_{k,h}^n(x) = h^{-\alpha } \sum _{\nu =0}^\alpha (-1)^\nu \binom {\alpha }{\nu } b_{k+\nu ,h}^{n-\alpha }\end{align*}errechnet werden.

Finite Elemente – B-Splines | Vorlesungsmitschriebe (2024)
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