Valter Moretti ha scritto:
> Ciao, quello che hai scritto � veramente molto interessante e non ne
> sapevo niente.
>
> Ci devo riflettere sopra (e fare l'esperimento).
Avevo continuato a chiedermi da dove avevo preso l'idea.
Pensa e ripensa, si e' accesa una lampadina: Martin Gardner!
E infatti: nel terzo volume delle "Mathematical Diversione from
Scientific American" c'e' un capitolo intitolato "Group Theory and
Braids" dove si trova una variante del giochetto. Ti cito due frasi:
"M.H.A. Newman, in an article published in a Londom mathematical
journal in 1942, says that P.A.M. Dirac [...] has for many years used
the solitaire form of this game as a model 'to illustrate the fact
that the fundamental group of the group of rotations in 3-space has a
single generator of period 2'."
"In a brief letter from Dirac that I received too late to mention in
the column on braids, he said that he had first thought of the string
problem about 1929, and had used it many times since to illustrate
that two rotations of a body about an axis con be continuously
deformed, through a set of motions each of which ends with the
original position, into no motion at all. 'It is a consequence' he
wrote 'of this property of rotations that a spinning body can have
half a quantum of angular momentum, but cannot have any other fraction
of a quantum'."
Questa cosa io l'avevo letta intorno al 1962, l'avevo adattata, e poi
avevo del tutto dimenticato l'origine, fino a oggi...
L'articolo cui si fa cenno e'
M.H.A. Newman: "On a String Problem of Dirac"; J. London Math. Soc. 17
(1942) part 3, 173.
--
Elio Fabri
Received on Sun Nov 30 2008 - 21:15:18 CET