Diffeological space, diffeological vector space, diffeologically smooth bilinear form
It is known that the only finite-dimensional diffeological vector space that admits a diffeologically smooth scalar product is the standard space of appropriate dimension. In this note, a way to dispense withthis issue is considered, by introducing a notion of pseudo-metric, which, said informally, is the least-degeneratesymmetric bilinear form on a given space. This notion is applied to make some observations on subspaces which split off as smooth direct summands (providing examples which illustrate that not all subspaces do), and then to show that the diffeological dual of a finite-dimensional diffeological vector space always has the standard diffeology and in particular, any pseudo-metric on the initial space induces, in theobvious way, a smooth scalar product on the dual.
"On the Notion of Scalar Product for Finite-dimensional Diffeological Vector Spaces",
Electronic Journal of Linear Algebra,
Volume 34, pp. 18-27.