| http://www.w3.org/ns/prov#value | - Besides the above-described preferred embodiments shown in FIGS. 4 and 5, as a pitch circle diameter ratio of the respective corrugating rolls 1 and 2, various ratios can be conceived, and if they are chosen among N:1 or M+L:M (N, M, L being integers equal to or larger than 1, N is an integer smaller than a number of teeth of the larger-diameter corrugating roll, and M+L is an integer smaller than
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