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K-theory and Noncommutative Geometry.pdf

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96 F. Muro and A. Tonks(R5 0 ) Œ D 0.This equivalence follows from (R8) on considering the diagrams A 1 A A 1 AA,A1 A A1 A A.We now introduce the basic object of study of this paper.Definition 1.5. A weak cofiber sequence in a Waldhausen category C is a diagramA B C 1Cgiven by a cofiber sequence followed by a weak equivalence in the opposite direction.We associate the elementŒA B C 1C D ŒA B C1 C ŒC ! C 1 ŒA 2 D C (1.1)to any weak cofiber sequence. By (R1) and (R2) we have@ŒA B C 1C D ŒB C ŒC C ŒA:The following fundamental identity for weak cofiber sequences is a slightly lesstrivial consequence of the relations (R1)–(R9).Proposition 1.6. Consider a commutative diagramA0 j AAr AA 000w AA 00j 0 B 0 j BjBr Bj 000 B 000w Bj 00B 00 r 00r 0 D 0 w 0 C 0 rr 000 w 000j D rD D D000 D 00j Cw Cr CC 000 w Dw 00 C 00w C

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