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{\bf Question}

Check the Henon map is invertible when $b \ne 0$; find a formula
for its inverse.
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{\bf Answer}

If $X=a-x^2+by,\ Y=x$ then $x=Y$ and $y=b^{-1}(X-a+Y^2)$: so the
inverse map to $f$ is $(X,Y) \mapsto (Y,b^{-1}(X-a+Y^2))$.
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