= Bäcklund generating current for sine-Gordon conserved charges
{c}
{title2=$\partial_\eta[(1-\cos d)/a^2]=\partial_\xi[1-\cos(u+d)]$}
Write the transformed angular field as $v=u+2d$. The <Sine-Gordon Bäcklund transformation> gives $\sin d=a(u_\xi+d_\xi)$, determining $d$ as a formal local derivative expansion in $a$. The displayed exact current identity yields a <conservation law> at each order. Formal convergence is unnecessary because each coefficient obeys an exact identity on solutions.
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