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Thursday, 22nd March 2001, 4:00 pm

Mathematics Bldg., lecture hall 2

Professor Marshall Cohen

(Cornell)

"Car Crashes, Group Extensions and s-Cobordisms"

** Abstract: **
We outline the proof of the following theorem, joint with Colin Rourke
(University of Warwick).

**THEOREM:** Suppose that *G* is a torsion free group and
(t) is the infinite cyclic group generated
by an element *t*. Let *w* be an element of the free product
*Gx(t)* and *((w))*
the normal subgroup generated by *w*. Then the natural map
* G -> G*(t) -> (G*(t))/((w))*
is onto iff *w* is conjugate to *gt* or
*gt^-1* for some *g* in *G*.

The proof uses Klyachko's ingenious car-crash method, used in the solution of the Kervaire conjecture. The theorem immediately applies topologically to the recognition of certain s-cobordisms.

Coffee, Cookies at the faculty lounge at 3:30.

You are invited to join the speaker for further discussion after the talk at Beit Belgia.

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