Abstract
We recall the notions of Frölicher and diffeological spaces, and we build regular Frölicher Lie groups and Lie algebras of formal pseudo-differential operators in one independent variable. Combining these constructions with a smooth version of Mulase’s deep algebraic factorization of infinite-dimensional groups based on formal pseudo-differential operators, we present two proofs of the well-posedness of the Cauchy problem for the Kadomtsev–Petviashvili (KP) hierarchy in a smooth category. We also generalize these results to a KP hierarchy modelled on formal pseudo-differential operators with coefficients which are series in formal parameters, we describe a rigorous derivation of the Hamiltonian interpretation of the KP hierarchy, and we discuss how solutions depending on formal parameters can lead to sequences of functions converging to a class of solutions of the standard KP-II equation.
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Notes
As explained in the previous paragraphs, our favoured setting is the category of Frölicher spaces. This category is a full subcategory of the category of diffeological spaces, and therefore it is natural to begin by considering the latter spaces, as we do in Sect. 2.
Jets are considered in [54], in the classical paper [7] and in the more recent review [47]; we note that in [54] the author introduces jets within the category DS of differential spaces, but it is known that Frölicher spaces form a subcategory of DS, see [32]. Moreover, as explained in Sect. 2.5 (see also [19, 32]) the category of Frölicher spaces is Cartesian closed, complete and cocomplete, and so infinite jets in Frölicher spaces can be defined by adapting the constructions of [7, 47].
The following example was essentially suggested by the referee: let us fix a smooth function \(u_0:S^1 \rightarrow { \! \mathrm \ I\!K}\) and think of it as a (trivial) element of \(R_z = C^\infty (S^1,{ \! \mathrm \ I\!K})[[z]]\). Then, we obtain a unique solution \(u = \sum \sum f_n^q(x,qt_1,q^2y,q^3t, qt_4, \cdots )z^n q^k\) to the q-deformed KP-II Eq. (5.19), with y-dependence of u fixed by integration. On the other hand, let us choose a smooth function f on \(S^1\) with \(f(0)=0\) and such that \({\tilde{u}}_0(x,q^2 y) = q u_0(x)+ q f(q^2 y)\) is different from \(u(x,0,q^2 y,0,\cdots )\). Then, the solution to KP-II with initial data \((1/q){\tilde{u}}_0\) arising via Bourgain’s theorem [9] induces, as explained in Sect. 4.2, a solution \({\tilde{u}}\) to our q-deformed KP-II equation. The function \({\tilde{u}}\) cannot coincide with our solution u, even though they coincide at \(t=y=0\).
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Both authors have been partially supported by CONICYT (Chile) via the Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT) operating grant # 1161691.
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Communicated by Nikolai Kitanine.
Dedicated to the memory of Professor Leonid Aleksandrovich Dickey.
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Magnot, JP., Reyes, E.G. Well-Posedness of the Kadomtsev–Petviashvili Hierarchy, Mulase Factorization, and Frölicher Lie Groups. Ann. Henri Poincaré 21, 1893–1945 (2020). https://doi.org/10.1007/s00023-020-00896-3
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DOI: https://doi.org/10.1007/s00023-020-00896-3