Celebratio Mathematica

F. Lalonde and D. McDuff: “Positive paths in the linear symplectic group,” pp. 361–387 in The Arnold–Gelfand mathematical seminars. Edited by V. I. Arnold, I. M. Gelfand, M. Smirnov, and V. S. Retakh. Birkhäuser (Boston), 1997. MR 1429901 Zbl 0868.58031 incollection

A positive path in the linear symplectic group \( \mathrm{Sp}(2n) \) is a smooth path which is everywhere tangent to the positive cone. These paths are generated by negative definite (time-dependent) quadratic Hamiltonian functions on Euclidean space. A special case are autonomous positive paths, which are generated by time-independent Hamiltonians, and which all lie in the set \( \mathcal{U} \) of diagonalizable matrices with eigenvalues on the unit circle. However, as was shown by Krein, the eigenvalues of a general positive path can move off the unit circle. In this paper, we extend Krein’s theory: we investigate the general behavior of positive paths which do not encounter the eigenvalue 1, showing, for example, that any such path can be extended to have endpoint with all eigenvalues on the circle. We also show that in the case \( 2n = 4 \) there is a close relation between the index of a positive path and the regions of the symplectic group that such a path can cross. Our motivation for studying these paths came from a geometric squeezing problem [Lalonde and McDuff, 1995] in symplectic topology. However, they are also of interest in relation to the stability of periodic Hamiltonian systems [Gelfand and Lidskii 1958] and in the theory of geodesics in Riemannian geometry [Bott 1956].

Yuriĭ Mikhaĭlovich Smirnov	Related
Vladimir Solomonovich Retakh	Related
Vladimir Igorevich Arnold	Related
François Lalonde	Related
Israïl Moiseevich Gelfand	Related

D. McDuff: “Symplectomorphism groups and quantum cohomology,” pp. 457–471 in The unity of mathematics: In honor of the ninetieth birthday of I. M. Gelfand (Cambridge, MA, 31 August–4 September 2003). Edited by P. Etingof, V. Retakh, and I. M. Singer. Progress in Mathematics 244. Birkhäuser (Boston), 2006. MR 2181814 Zbl 1099.53057 incollection

Vladimir Solomonovich Retakh	Related
Pavel Ilyich Ètingof	Related
Israïl Moiseevich Gelfand	Related
Isadore M. Singer	Related	Volume

D. McDuff, S. Tabachnikov, and M. Saul: “Israel Moiseevich Gelfand, Part II.” Edited by V. Retakh. Notices Am. Math. Soc. 60 : 2 (2013), pp. 162–171. Zbl 1290.01026 article

Vladimir Solomonovich Retakh	Related
Israïl Moiseevich Gelfand	Related
Mark E. Saul	Related
Serge Lvovich Tabachnikov	Related

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