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The PL hierarchy collapses. (English)
SIAM J. Comput. 27, No.5, 1430-1437 (1998).
Summary: It is shown that the PL hierarchy $\text{PLH}= \text{PL}\cup\text{PL}^{PL}\cup \text{PL}^{PLPL}\cup\cdots$, defined in terms of the Ruzzo-Simon-Tompa relativization, collapses to PL.