nLab Michal-Bastiani smooth map



One can ask whether a nonlinear map between locally convex topological vector spaces is smooth according to various definitions. The definition due to Michal, developed by Bastiani, is a very general such notion.


A map Φ:EF\Phi:E\rightarrow F from a locally convex topological vector space (lctvs) EE into another lctvs FF is said to be Michal-Bastiani smooth (or MB-smooth) if its directional derivatives of order kk,

D kΦ[x](y 1,,y k)= kλ 1λ k| λ 1==λ k=0Φ(x+ j=1 kλ jy j) D^k\Phi[x](y_1,\ldots,y_k) = \left.\frac{\partial^k}{\partial\lambda_1\cdots\partial\lambda_k}\right|_{\lambda_1=\cdots=\lambda_k=0}\Phi\left(x+\sum^k_{j=1}\lambda_j y_j\right)

exist and the maps D kΦ:E×E kFD^k\Phi:E\times E^k\rightarrow F are jointly continuous? for all kk\in\mathbb{N}. The notion is due to Michal (Michal 38,Michal 40), and was further developed by (Bastiani 64).

This notion of smoothness is the one used in Milnor‘s treatment of infinite-dimensional Lie groups (Milnor 84) and Hamilton’s exposé of the Nash-Moser inverse function theorem (Hamilton 92). A comparison to other notions of smoothness for topological vector spaces with various properties is in (Keller 74).


MB-smooth maps have the following properties.

Note that a map that is smooth in the sense of a convenient vector space (equivalently, a smooth map between the corresponding diffeological spaces) is not necessarily continuous, so not all convenient smooth maps between lctvs are MB-smooth. Explicit examples have been given in (Glöckner 06). It is even the case that one can have a convenient isomorphism that is not MB-smooth, so the faithful and non-full functor

lctvs MBlctvs convenientDiffeologicalSpace lctvs_{MB} \to lctvs_{convenient} \hookrightarrow DiffeologicalSpace

is not even injective on isomorphism classes. The following example was supplied in (TaQ 15):

Let EE be the vector space of all real (two-sided) sequences x=spx iix=\langle\sp x_i\mid i\in\mathbb{Z}\rangle for which x i=0x_i=0 for i0i\ll 0, topologized so that we get a linear homeomorphism E × ()E\to\mathbb{R}^\mathbb{N}\times\mathbb{R}^{(\mathbb{N})} defined by x(u,v)x\mapsto(u,v) where u i=x i1u_i=x_{i-1} and v i=x iv_i=x_{-i} for i\gt 0}.

Then define the bijection f:EEi\gt 0}f:E\to E by xyx\mapsto y where y i=x iy_i=x_i for i0i\neq 0 and y 0=x 0+ i(x ix i)y_0=x_0+\sum_{i\in\mathbb{N}}(x_i\cdot x_{-i}). The inverse is given by yxy\mapsto x where y i=x iy_i=x_i for i0i\neq 0 and x 0=y 0 i(y iy i)x_0=y_0-\sum_{i\in\mathbb{N}}(y_i\cdot y_{-i}). The duality map × ()\mathbb{R}^\mathbb{N}\times\mathbb{R}^{(\mathbb{N})}\to\mathbb{R}, x i(x ix i)x\mapsto \sum_{i\in\mathbb{N}}(x_i\cdot x_{-i}), is discontinuous but bornological, and hence a conveniently smooth bilinear map. Thus ff is discontinuous and so not MB-smooth.


  • Aristotle Demetrius Michal, Differential calculus in linear topological spaces, Proc. Nat. Acad. Sci. USA 24 (1938), 340-342 (pdf)

  • Aristotle Demetrius Michal, Differential of functions with arguments and values in topological abelian groups, Proc. Nat. Acad. Sci. USA 26 (1940), 356–359. (pdf)

  • Andree Bastiani, Applications différentiables et variétés différentiables de dimension infinie, J. Anal. Math. 13 (1964), 1–114. (article, paywalled)

  • John Milnor, Remarks on Infinite-Dimensional Lie Groups, in: B. DeWitt, R. Stora, eds., Les Houches Session XL, Relativity, Groups and Topology II (North-Holland, 1984), pp. 1007-1057

  • Richard Hamilton, The Inverse Function Theorem of Nash and Moser. Bull. Amer. Math. Soc. 7 (1982) 65-222.

  • Keller, H. H., Differential Calculus in Locally Convex Spaces, Lecture Notes in Mathematics 417, Springer-Verlag, 1974 (Springerlink, paywalled)

  • Helge Glöckner?, Discontinuous non-linear mappings on locally convex direct limits, Publ. Math. Debrecen 68 (2006) 1-13, arXiv:math/0503387.

  • TaQ (user 12643), Answer to Do locally convex topological vector spaces embed into diffeological spaces?, MathOverflow, <> (version: 2015-06-16)

Last revised on June 17, 2015 at 03:08:42. See the history of this page for a list of all contributions to it.