3: Knots and Algebra

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Learning Objectives

Calculate algebraic structures that are invariants for knots, including the fundamental quandle and the knot group, and use these to distinguish among knots.
Calculate polynomials that are invariants for knots, including the Alexander, HOMFLY, and Jones polynomials, and use these to distinguish among knots.

Numerical invariants such as we studied in Chapter \(2\) are powerful, especially insofar as they can help us to discover relationships among different classes of knots. But while the Conway notation comes closest, none of these numerical invariants can carry enough information about the type of a knot to permit us to surely distinguish one knot from another. While they are sensitive (different numbers imply different knots), they are far from specific (same number does not imply same knot).

In our search for a complete invariant for knots, then, we need a structure capable of holding more information than can a single number. In this culminating chapter we'll work our way through algebraic structures of increasing complexity, each of which adds much-needed specificity. The rogue's gallery of structures range from the familiar (polynomials) to the recognizable (groups) to the exotic (quandles), each with its own advantages and limitations. But reckoning with how each one encodes information about a knot is the key to understanding the role of algebraic thinking in knot theory in particular, and in mathematics more generally.

References

Adams, C. C. (2004). The Knot Book: an elementary introduction to the mathematical theory of knots. American Mathematical Society, ISBN 0-8218-3678-1. Chapter 5.
Austin, D. (2016). Knot quandaries quelled by quandles. American Mathematical Society Feature Column, accessed at http://www.ams.org/publicoutreach/feature-column/fc-2016-03.
Rolfsen, D. (1990). Knots and Links. Corrected reprint of the 1976 original. Mathematics Lecture Series (7). American Mathematical Society. Chapter 3.