Jacobian of rhs

docs/nstrophy_doc.tex

@@ -407,12 +407,11 @@ and, by\-~(\ref{T}),
     \frac{(\ell\cdot q^\perp)|\ell|}{|q|}
   \right)\hat u_q
   =
-  \sum_{\displaystyle\mathop{\scriptstyle q\in\mathbb Z^2}_{\ell+q=k}}
-  (q\cdot \ell^\perp)\left(
-    \frac{|q|}{|\ell|}
+  (k\cdot \ell^\perp)\left(
+    \frac{|k-\ell|}{|\ell|}
     -
-    \frac{|\ell|}{|q|}
-  \right)\hat u_q
+    \frac{|\ell|}{|k-\ell|}
+  \right)\hat u_{k-\ell}
   .
 \end{equation}
 \bigskip