process

146 M. Karoubiwhere n is the ring of n-cyclotomic integers. We can show that this operation isadditive and multiplicative up to canonical isomorphisms (see Theorem 30, p. 23 in[19]).We should also notice, following [9], that we can define the Adams operation ‰ 1in graded twisted K-theory as well and combine it with the ‰ n ’s in order to define“internal” operations from K A .X/ to K A y˝n.X/ ˝ n .ZThe simplest non-trivial example is‰ n W Z Š K 1 .S 1 / ! K n .S 1 / ˝Z n Š K 1 .S 1 / ˝ nZwhere n is a product of different odd primes. Since the operation ‰ n on K 2 .S 2 / is themultiplication by n, we deduce that D p . 1/ .n 1/=2 n belongs to n (a well-knownresult due to Gauss) and that ‰ n on K 1 .S 1 / is essentially the inclusion of Z in ndefined by 1 7! .As a concluding remark, we should notice that the image of ‰ n as defined in 7.8 isnot arbitrary. If k and n are coprimes, the multiplication by k on the group Z=n definesan element of the symmetric group S n . The signature of this permutation is called theLegendre symbol . k n /. Moreover, this permutation conjugates the elements r and rk inthe group Z=n. If the Legendre symbol is 1, this permutation can be lifted to the Schurgroup C n . Let us denote now by F r (as in 7.4) the element of the twisted K-groupassociated to the eigenvalue e 2ir . Then we see that F r and F rk are isomorphic ifthe Legendre symbol . k n / is equal to 1 since r and rk are conjugate by an element ofthe Schur group. If n is prime for instance, ‰ n .E/ may therefore be written in thefollowing way:‰ n .E/ D F 0 C XU! k C XV! k ;. k n/D1. k n/D 1where U (resp. V )isanyF k with Legendre’s symbol equal to 1 (resp. 1). Thisshows in particular that the element D p . 1/ .n 1/=2 n in 7.9 is a “Gauss sum”, awell-known result.8 Appendix: A short historical survey of twisted K-theoryTopological K-theory was of course invented by Atiyah and Hirzebruch in 1961 [4]after the fundamental work of Grothendieck [14] and Bott [15]. However, twistedK-theory which is an elaboration of it took some time to emerge. One should quotefirst the work of Atiyah, Bott and Shapiro [6] where Clifford modules and Cliffordbundles (in relation with the Dirac operator) where used to reinterpret Bott periodicityand Thom isomorphism in the presence of Spin or Spin c structures. We then started tounderstand in [28], as quoted in the introduction, that K-theory of the Thom space maybe defined almost algebraically as K-theory of a functor associated to Clifford bundles.Finally, it was realized in [19] that we can go a step further and consider general gradedalgebra bundles instead of Clifford bundles associated to vector bundles (with a suitablemetric).