* Dinatural transformations
@ 2004-07-05 6:35 Max Kelly
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From: Max Kelly @ 2004-07-05 6:35 UTC (permalink / raw)
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Nason Yanofsky asked a question about composition of dinatural
transformations, and there have been a number of replies - but none
giving the reference I should have expected, namely [S.Eilenberg and
G.M.Kelly, A generalization of the functorial calculus,, J.Algebra 3
(1966), 366 - 375] .
What Sammy and I were concerned with were such families as the
evaluationn e_A,B : [A,B] o A --> B , where o is a tensor product and [
, ] is an internal hom. Here e_A,B is natural in B in the usual sense;
it is also natural, in our extended sense, in the variable A, which
appears twice on one side of the arrow, but with two opposite variances.
Similar for d_A,B : A --> [B, AoB]. In certain circumstances one can
compose such "natural transformations" to form new ones:one example is
the composite
AoB ---------> [B, AoB] o B ----------> AoB
d_A,B o B e_B, AoB
which is in fact the identity natural transformation.
Sammy and I gave a general treatment in the article above. Later, others
generalised our "extended naturals" to get the notion of dinatural
transformation. Since these do not compose except in some very special
cases, I find them to be of limited interest. In contrast, I find that
I still use the Eilenberg-Kellycalculus from time to time.
Max Kelly.
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