A^1-同伦群、切除与可解同伦
A^1-homotopy groups, excision, and solvable quotients
摘要：
本文研究了A^1-同伦群的一些性质：连通性的几何解释，切除结果，以及A^1-同伦理论意义下连通可解群在覆盖空间上的自由作用对同伦项的重新解释.这些概念和结果非常适合于通过几何不变量理论来研究某些连续性。作为可解群簇几何的一个研究案例，我们研究光滑环面簇的A^1-同伦群。我们给出了保证光滑（真）复曲面簇的低次A^1-同伦群为零的简单组合条件（用扇形表示）。最后，在某些情况下，我们实际上可以计算光滑复曲面簇的“下一个”非零A^1-同伦群（超过π_1^{A^1}）。从这个观点来看，A^1-同伦理论，即使对代数几何结构极其敏感，对于大类有趣的簇来说，几乎和普通同伦一样“容易处理”（在低次上）。
英文摘要：
We study some properties of A^1-homotopy groups: geometric interpretations of connectivity, excision results, and a re-interpretation of quotients by free actions of connected solvable groups in terms of covering spaces in the sense of A^1-homotopy theory. These concepts and results are well-suited to the study of certain quotients via geometric invariant theory. As a case study in the geometry of solvable group quotients, we investigate A^1-homotopy groups of smooth toric varieties. We give simple combinatorial conditions (in terms of fans) guaranteeing vanishing of low degree A^1-homotopy groups of smooth (proper) toric varieties. Finally, in certain cases, we can actually compute the "next" non-vanishing A^1-homotopy group (beyond π_1^{A^1}) of a smooth toric variety. From this point of view, A^1-homotopy theory, even with its exquisite sensitivity to algebro-geometric structure, is almost "as tractable" (in low degrees) as ordinary homotopy for large classes of interesting varieties.
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