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On the Product of Real Spectral Triples
Authors:Vanhecke  F J
Institution:(1) Instituto de Física, UFRJ, Ilha do Fundão, Rio de Janeiro, Brasil
Abstract:The product of two real spectral triples 
$$\left\{ {\mathcal{A}_1 ,\mathcal{H}_1 ,\mathcal{D}_1 ,\mathcal{J}_1 ,{\gamma }_{\text{1}} } \right\} $$
and 
$$\left\{ {\mathcal{A}_2 ,\mathcal{H}_2 ,\mathcal{D}_2 ,\mathcal{J}_2 ,\left( {{\gamma }_{2} } \right)} \right\} $$
, the first of which is necessarily even, was defined by A.Connes as 
$$\left\{ {\mathcal{A},\mathcal{H},\mathcal{D},\mathcal{J}\left( {,{\gamma }} \right)} \right\} $$
given by 
$$\mathcal{A} = \mathcal{A}_1  \otimes \mathcal{A}_2 ,\mathcal{H} = \mathcal{H}_1  \otimes \mathcal{H}_2 ,\mathcal{D} = \mathcal{D}_1  \otimes {\text{Id}}_{\text{2}}  + {\gamma }_{1}  \otimes \mathcal{D}_2 ,\mathcal{J} = \mathcal{J}_1  \otimes \mathcal{J}_2 $$
and, in the even-even case, by 
$$ {\gamma }_{1}  \otimes {\gamma }_{2} $$
. Generically it is assumed that the real structure 
$$\mathcal{J} $$
obeys the relations 
$$ \mathcal{J}^2  = \varepsilon {\text{Id}} $$
, 
$$ \mathcal{J}\mathcal{D} = \varepsilon '\mathcal{D}\mathcal{J} $$
, 
$$ \mathcal{J}{\gamma  = }\varepsilon '{\gamma }\mathcal{J} $$
, where the 
$$ \varepsilon $$
-sign table depends on the dimension n modulo 8 of the spectral triple. If both spectral triples obey Connes' 
$$ \varepsilon $$
>-sign table, it is seen that their product, defined in the straightforward way above, does not necessarily obey this 
$$ \varepsilon $$
-sign table. In this Letter, we propose an alternative definition of the product real structure such that the 
$$ \varepsilon $$
-sign table is also satisfied by the product.
Keywords:noncommutative geometry  real spectral triples
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