From: "C. Hillman" <optimist@u.washington.edu>
Newsgroups: sci.math
Subject: Re: Twisted phone cords
Date: Sat, 27 Jun 1998 09:30:51 -0700

On Sat, 27 Jun 1998, physical (Droll Troll) wrote:

> Being only familiar with the DNA formula (L=T+W, I think),

That's White's formula in differential topology, which has been -applied-
to a "closed" (or "circular") chromosome consisting of two strands of DNA
(Many bacteria have a single such chromosome).  We can model the topology
of the two DNA strands with a "closed" ribbon R.  The twist Tw(R), while
the writhe Wr(R)  measures the contortion of the axis of the ribbon in
space.  White's formula says that the linking number Lk(R) of two edges is
the sum of the twist Tw(R) and the writhe Wr(R):

  Lk(R) = Tw(R) + Wr(R)

See Colin Adams, The Knot Book, Freeman, 1994, for an application to (!!!)
the supercoiling of phone cords as well as the DNA application.

Chris Hillman

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Thanks!