17

Upon discussing this with Michael T. Goodrich, it seems that the parallel sorting algorithm by Cole for EREW PRAM does the job. See Richard Cole: "Parallel Merge Sort". SIAM J. Comput. 17(4): 770-785 (1988). In that algorithm there are $O(\log n)$ rounds and in each round each element participates in $O(1)$ comparisons. (One has to understand the algorithm ...


16

How do you decide what the "wrong way" is? Take the first wrong-way swap gate, and interchange the two wires going out of it (including all their associated gates) so that it's correct. This doesn't change the fundamental circuit. It may introduce more wrong-way swap gates, but they're all later in the circuit. Now, you can keep doing this until you've ...


15

I think that whether p>0 can be decided in polynomial time. The problem in question can be easily cast as the edge-disjoint paths problem, where the underlying graph is a planar graph consisting of m+1 layers each of which contains n vertices, plus m degree-4 vertices to represent the possible adjacent swaps. Note that the planarity of this graph ...


11

$\def\et{\mathbin\&}$Yes, and this holds much more generally. Note that a comparator can be thought of as a pair of gates, one of which computes $\min\{x,y\}$, and the other $\max\{x,y\}$. A linearly ordered set is a distributive lattice with $x\land y=\min\{x,y\}$ and $x\lor y=\max\{x,y\}$. We have the following 0–1 principle: Let $C$ be a circuit with ...


8

It seems not. Ian Parberry makes reference to a paper by Chung and Ravikumar, where they supposedly give a recursive construction of a sorting network that sorts every bitstring but one, and further deduce that the problem of verifying a sorting network is $co$-$NP$ complete. I can't find the original paper right away, but certainly it matches (my) ...


7

No, bitonic sort is not stable. For this post I will denote numbers as 2;0 where only the part before the ; is used for comparison and the part behind ; to mark the initial position. Comparison-exchanges are denoted by arrows where the head points at the desired location of the greater value. As written in the link that @JukkaSuomela posted a stable ...


5

No, such an algorithm cannot exist. Assume $t$ comparisons per element are allowed. For a start, consider the situation of merging two lists, one of size one, and the other of size $2^t$. There are $2^t+1$ possible results, and an easy adversarial argument shows that the element in the small list needs to participate in $t+1$ comparisons, and this element ...


3

Here's some empirical data for question 2, based on D.W.'s idea applied to bitonic sort. For $n$ variables, choose $j - i = 2^k$ with probability proportional to $\lg n - k$, then select $i$ uniformly at random to get a comparator $(i,j)$. This matches the distribution of comparators in bitonic sort if $n$ is a power of 2, and approximates it otherwise. ...


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