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Tate sphere

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Idea

The Tate sphere, in the strict sense, is the object in a sheaf topos (or (∞,1)-sheaf (∞,1)-topos, or its stabilization) over a gros site of algebraic varieties, which is one of the following: 1\mathbb{P}^{1} (the projective line); the quotient

S Tate 1𝔸 1/(𝔸 1{0}), S^1_{Tate} \;\coloneqq\; \mathbb{A}^1 / (\mathbb{A}^1 \setminus \{0\}) \,,

which denotes the (homotopy-)cofiber of the inclusion morphism into the sheaf represented by the affine line 𝔸 1\mathbb{A}^1 from the subobject 𝔸 1{0}\mathbb{A}^1 \setminus \{0\} represented by the affine line with origin removed (equivalently the object underlying the multiplicative group 𝔾 m\mathbb{G}_m); or S 1𝔾 mS^{1} \wedge \mathbb{G}_{m}.

These constructions are understood by default in the context of motivic homotopy theory working over the Nisnevich site (VRO 07, Remark 2.22), where they are all 𝔸 1\mathbb{A}^{1}-homotopy equivalent. But the construction principle, especially that of the homotopy cofibre definition, is clearly more general.

In formal constructions of categories of motives, one typically ‘inverts the Tate sphere’ in some sense in order to represent Tate twists correctly in Weil cohomology theories such as étale cohomology, or in motivic cohomology theories. This is the case, for example, both for the stable motivic homotopy categories à la Voevodsky, where one more precisely inverts the operation of smashing with the Tate sphere, and for the category of pure motives, where one inverts the Lefschetz motive.

References

Last revised on August 3, 2020 at 06:40:41. See the history of this page for a list of all contributions to it.