Register to reply

Retraction in surface of genus g

by ForMyThunder
Tags: genus, retraction, surface
Share this thread:
Jun29-11, 05:25 PM
P: 152
1. The problem statement, all variables and given/known data
In the surface Mg of genus g, let C be a circle that separates Mh' and Mk' obtained from the closed surfaces Mh and Mk by deleting an open disk from each. Show that Mh' does not retract onto its boundary circle C, and hence Mg does not retract onto C.

Hatcher Allen. Algebraic Topology Section 1.2 Problem 9

2. Relevant equations

3. The attempt at a solution Suppose there was such a retraction. Then we would have that [tex]i_*:\pi_1(C)\to\pi_1(M'_h)[/tex] induced by the inclusion map is injective and that [tex]\phi:\pi_1(M'_h)\to\pi_1(M_h)[/tex] is surjective with kernel [itex]i_*(\pi_1(C))[/itex]. Thus, [tex]\pi_1(M'_h)/i_*(\pi_1(C))\cong \pi_1(M_h)[/tex] and by taking the abelianizations: [tex]\mathbb{Z}^{2h-1}\cong\mathbb{Z}^{2h}/\mathbb{Z}\cong\mathbb{Z}^{2h}[/tex] yielding a contradiction.

Is this correct? I used the assumption that C was a retract of Mh' to say that the fundamental group of C is isomorphic to a subgroup of the fundamental group of Mh'.
Phys.Org News Partner Science news on
New type of solar concentrator desn't block the view
Researchers demonstrate ultra low-field nuclear magnetic resonance using Earth's magnetic field
Asian inventions dominate energy storage systems
Jun30-11, 05:13 PM
P: 152
Actually, in that last line, [itex]\mathbb{Z}^{2h-1}\cong\mathbb{Z}^{2h}/\mathbb{Z}[/itex] is not necessarily true. But the second isomorphism in that line is implied by the previous line and this leads to the contradiction.

Register to reply

Related Discussions
Projective planes and retraction Calculus & Beyond Homework 0
Scapula retraction and protraction Medical Sciences 6
Retraction/reflection help please Introductory Physics Homework 9
Retraction math help Linear & Abstract Algebra 32
Why not a quantum of any topological genus? Quantum Physics 6