Richard Beigel, William Gasarch, Efim Kinber

For a set A and a number n let F_n^A(x_1,...,x_n) =

A(x_1)\cdots A(x_n). We study how hard it is to approximate this

function in terms of the number of queries required. For a general

set A we have exact bounds that depend on functions from coding

theory. These are applied ...
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Yijia Chen, Jörg Flum

In [Blass, Gurevich, and Shelah, 99] a logic L_Y has been introduced as a possible candidate for a logic capturing the PTIME properties of structures (even in the absence of an ordering of their universe). A reformulation of this problem in terms of a parameterized halting problem p-Acc for nondeterministic ... more >>>

Scott Aaronson, Mohammad Bavarian, Giulio Gueltrini

We ask, and answer, the question of what's computable by Turing machines equipped with time travel into the past: that is, closed timelike curves or CTCs (with no bound on their size). We focus on a model for CTCs due to Deutsch, which imposes a probabilistic consistency condition to avoid ... more >>>