群上同调中的本质理想不平方为零
The essential ideal in group cohomology does not square to zero
摘要：
设G是酉群$SU_3(4)$的Sylow 2-子群.在G的mod-2上同调环中找到了乘积非零的两个本质类.事实上，产品是本森-卡尔森二元性的“最后幸存者”。Pakianathan和Yalcin最近的工作则暗示了关于作用为G的连通图的一个结果。此外，存在本质类，不能被写为从真子群转移的和。这种现象是在计算机上首次发现的。这里给出的论证使用了克拉克（J. Clark）的精确计算，并做了一些小的修正。
英文摘要：
Let G be the Sylow 2-subgroup of the unitary group $SU_3(4)$. We find two essential classes in the mod-2 cohomology ring of G whose product is nonzero. In fact, the product is the ``last survivor'' of Benson-Carlson duality. Recent work of Pakianathan and Yalcin then implies a result about connected graphs with an action of G. Also, there exist essential classes which cannot be written as sums of transfers from proper subgroups. This phenomenon was first observed on the computer. The argument given here uses the elegant calculation by J. Clark, with minor corrections.
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