不可解问题的判定以特定的不可解性为其研究对象，是数理逻辑的重要研究领域之一。
这样一个问题甚至可以难到最聪明的人，对年轻的亚瑟王来说，这就像一个不可解的问题。
Such a question would perplex even the most knowledgeable man, and, to young Arthur, it seemed an impossible query.
理论上已经证明要在多项式时间复杂度内对这一类问题找到全局最优解是不可能的。
The fact that it is impossible to find the global optimum in polynomial complexity has been proved.
辫群是一种非交换的无限群，该群中有许多困难问题是不可解的，如字问题、共轭问题和根问题等，利用这些困难问题可以去设计一些密码协议。
The braid group is infinite non-commutative group, and it has many hard problems that can be utilized to design cryptographic primitives, such as the word problem, conjugacy problem and root problem.
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