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Sossinsky:Knots. Mathematics with a twist.pdf - English

Sossinsky:Knots. Mathematics with a twist.pdf - English

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KNOTS<br />

Figure 1.3. Planar projection of a knot.<br />

specifying which knots belong to the same class. It requires, in other<br />

words, precisely defining the equivalence of knots. Sut we will leave this<br />

definition (the ambient isotopy of knots) for later, limiting ourselves<br />

here to an intuitive description. Imagine that the curve defining the<br />

knot is a fine thread, flexible and elastic, that can be <strong>twist</strong>ed and<br />

moved in a continuous way in space (cutting and gluing back is not al­<br />

lowed). AlI possible positions will thus be those of the same knot.<br />

Changing the position of the curve that defines a knot in space by<br />

moving it in a continuous way (<strong>with</strong>out ever cutting or retying it) al­<br />

ways results in the same knot by definition, but its planar representa­<br />

tion may become unrecognizable. In particular, the number of cross­<br />

ings may change. Nevertheless, the natural approach to classifying

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