| 摘要 |
A sheath is provided with necessary conditions where inner and outer surfaces thereof are both formed in an uneven helical shape, where the outer surface of a trough is almost flat, where a width W of the trough between inflection points of a curve along the inner surface of the trough and a width w of a top between inflection points of a curve along the top satisfy the relation of W>w, where a thickness T of the trough and a thickness t of the top satisfy the relation of T>t, where a projection of the inner surface of the trough is formed in a gentle arc compared to a recess of the inner surface of the top, and where an inner perimeter of the recess of the inner surface projects circumferentially outward more than an outer-perimeter of the outer surface of the trough. The sheath is entirely formed of polyolefin resin material.
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