The complexity of parallel comparison merging

We prove a worst case lower bound of Ω(log log n) for randomized algorithms merging two sorted lists of length n in parallel using n processors on Valiant´s parallel computation tree model. We show how to strengthen this result to a lower bound for the expected time taken by any algorithm on the uniform distribution. Finally, bounds are given for the average time required for the problem when the number of processors is less than and greater than n.