next up previous contents
Next: Lichnerowicz definition of the Up: Schouten-Nijenhuis bracket Previous: Schouten-Nijenhuis bracket   Contents

Schouten-Nijenhuis bracket computations

Fix a system of local coordinates and consider two vector fields

\begin{displaymath}
X=\sum_i a_i \del_{x_i},\quad Y=\sum_i b_i\del_{x_i},\quad
X,Y\in\gX(M),\quad x_1,\hdots, x_n\text{ coordinates}
\end{displaymath}


\begin{displaymath}[X, Y]=\sum_i a_i\left(\sum_j \pd{b_j}{x_i}\del_{x_j}\right)- \sum_i
b_i\left(\sum_j\pd{a_j}{x_i}\del_{x_j}\right).
\end{displaymath}

Let $\zeta_i=\del_{x_i}$ and consider it as an odd formal variable

\begin{displaymath}
\zeta_i\zeta_j=-\zeta_j\zeta_i\quad
(\del_{x_i}\land\del_{x_j}=-\del_{x_j}\land\del_{x_i}).
\end{displaymath}

Then

\begin{displaymath}
X:=\sum_i a_i \zeta_i,\quad Y:=\sum_i b_i\zeta_i
\end{displaymath}


\begin{align*}[X, Y]&=\sum_i\left(\pd{X}{\zeta_i}\pd{Y}{x_i}-
\pd{Y}{\zeta_i}\pd...
...\ &= \left(\sum_i
\del_{\zeta_i}\land\del_{x_i}\right)(X\tensor Y).
\end{align*}
Extend this idea to multivector fields

\begin{displaymath}
P\in\gX^p(M),\quad
P=\sum_{i_1<\hdots<i_p}\del_{x_{i_1}}\wed...
...i_1<\hdots<i_p} P_{i_i\hdots i_p}\zeta_{i_1}\hdots\zeta_{i_p}.
\end{displaymath}

Fix the following differentiation rule
\begin{align*}
\del_{\zeta_{i_p}}(\zeta_{i_1}\hdots\zeta_{i_p})
&=\zeta_{i_1}\hd...
...(-1)^{p-k}\zeta_{i_1}\hdots\Hat{\zeta_{i_k}}\hdots \zeta_{i_{p-1}}.
\end{align*}
Then we claim that

\begin{displaymath}[P, Q]_{SN}=\sum_{i}\del_{\zeta_i}P\,\del_{x_i}Q -
(-1)^{(p-1)(q-1)}\del_{\zeta_i}Q\,\del_{x_i}P.
\end{displaymath}



Pawel Witkowski 2006-06-26