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Let C be the topological knot concordance group of knots S1 ⊂ S3 under
connected sum modulo slice knots. Cochran, Orr and Teichner defined a
filtration:
The quotient C ∕ F(0.5) is isomorphic to Levine’s algebraic concordance group; F(0.5)
is the algebraically slice knots. The quotient C ∕ F(1.5) contains all metabelian
concordance obstructions.
Using chain complexes with a Poincaré duality structure, we define an abelian
group AC2, our second order algebraic knot concordance group. We define a group
homomorphism C→AC2 which factors through C ∕ F(1.5), and we can extract
the two stage Cochran–Orr–Teichner obstruction theory from our single
stage obstruction group AC2. Moreover there is a surjective homomorphism
AC2 →C ∕ F(0.5), and we show that the kernel of this homomorphism is
nontrivial.
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