 Reciprocal coordinate at infinite coupling

An infinite positive coupling coordinate $v$ can be studied through $w=1/v$, with $v=\infty$ represented by $w=0$. For example, $v^{\prime }=v^4/A(u)$ with $A(u)>0$ becomes $w^{\prime }=A(u)w^4$, a smooth map at zero. This gives a well-defined boundary renormalization-group fixed point whose derivative in the reciprocal direction is zero. One should use this chart rather than substituting infinity into an ordinary finite-coordinate Jacobian.