From Phoebe Young, Alford, Lincolnshire, UK
Tom Roberts asks whether an extension cord plugged into itself can be manipulated to form a knot (Letters, 12 September). We don’t need an equation or extra dimensions to solve his problem (and in fact, knots can only exist in 3D), but to just consider the definitions.
In knot theory, we don’t care how much you distort a knot as long as it isn’t cut and rejoined, so this extension cord loop, known as a trivial knot, stays trivial as long as it isn’t unplugged. This doesn’t stop it being tangled, however, which presents a key issue in knot theory – that of determining whether a given tangle can be unravelled to a trivial knot.
