= Retraction onto a punctured oriented surface
{title2=$\Sigma_h^\partial\subset\Sigma_g\text{ is a retract}\iff2h\leq g$}
A once-punctured genus-$h$ <closed orientable surface> retracts to a wedge of $2h$ circles. A <retraction> from $\Sigma_g$ would inject its degree-one <cohomology> as an <isotropic subspace of a symplectic vector space> for the <Poincare duality pairing>, since the punctured surface has zero degree-two cohomology. The maximal isotropic dimension is $g$, giving $2h\leq g$. Equality is realized by folding the double of the punctured surface onto one copy. Additional handles in the other copy can be pinched away while fixing the joining boundary, proving sufficiency for every embedding of such a one-boundary subsurface when $g\geq2h$.
Back to article page
