{"id":2400,"date":"2019-03-25T22:11:41","date_gmt":"2019-03-26T02:11:41","guid":{"rendered":"http:\/\/blogs.ams.org\/beyondreviews\/?p=2400"},"modified":"2019-03-25T22:11:41","modified_gmt":"2019-03-26T02:11:41","slug":"karen-uhlenbeck-wins-the-2019-abel-prize","status":"publish","type":"post","link":"https:\/\/blogs.ams.org\/beyondreviews\/2019\/03\/25\/karen-uhlenbeck-wins-the-2019-abel-prize\/","title":{"rendered":"Karen Uhlenbeck wins the 2019 Abel Prize"},"content":{"rendered":"<p><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-2401 alignleft\" src=\"http:\/\/blogs.ams.org\/beyondreviews\/files\/2019\/03\/Uhlenbeck-Abel-Prize-300x262.png\" alt=\"Official announcement of Abel Prize being awarded to Karen Uhlenbeck\" width=\"300\" height=\"262\" srcset=\"https:\/\/blogs.ams.org\/beyondreviews\/files\/2019\/03\/Uhlenbeck-Abel-Prize-300x262.png 300w, https:\/\/blogs.ams.org\/beyondreviews\/files\/2019\/03\/Uhlenbeck-Abel-Prize.png 467w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/MRAuthorID\/175775\">Karen Uhlenbeck<\/a> is being awarded the <a href=\"http:\/\/www.abelprize.no\/nyheter\/vis.html?tid=74161\">2019 Abel Prize<\/a>.\u00a0 It is a remarkable award for a remarkable mathematician.\u00a0 Uhlenbeck did fundamental work in a quickly developing area of mathematics at an early stage of its development.\u00a0 I was a graduate student when some of her significant papers were coming out.\u00a0 There were quite a few people trying to understand her results and her techniques, diving deep into the difficult analysis she was unleashing on geometric problems.\u00a0 Geometric analysts have long recognized Uhlenbeck&#8217;s contributions.\u00a0 It is nice to see Uhlenbeck and her work recognized more widely with the Abel Prize.\u00a0\u00a0<!--more--><\/p>\n<p>For an insightful description of some of Uhlenbeck&#8217;s work, the recent <a href=\"http:\/\/www.ams.org\/journals\/notices\/201903\/rnoti-p303.pdf\">article <\/a>\u00a0by <a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/MRAuthorID\/59010\">Simon Donaldson<\/a> in the <a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?id=400\"><em>Notices of the AMS<\/em> <\/a>is tremendous.\u00a0 \u00a0Below are the texts of some reviews of her work in MathSciNet.<\/p>\n<hr \/>\n<p><a href=\"https:\/\/mathscinet.ams.org\/mathscinet-getitem?mr=264714\"><strong>MR0264714<\/strong><\/a><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=175775\">Uhlenbeck, K.<\/a><br \/>\n<span class=\"title\">Harmonic maps; a direct method in the calculus of variations.<\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?journalKey=bull_amer_math_soc\"><em>Bull. Amer. Math. Soc.<\/em><\/a>\u00a0<strong>76\u00a0<\/strong>1970 1082\u20131087.<\/p>\n<p class=\"review\">It was shown by J. H. Sampson and the reviewer [Amer. J. Math.\u00a0<span class=\"bf\">86<\/span>\u00a0(1964), 109\u2013160;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=164306&amp;loc=fromrevtext\">MR0164306<\/a>] that in every homotopy class of maps of one compact Riemannian manifold\u00a0<span id=\"MathJax-Element-1-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-1\" class=\"math\"><span id=\"MathJax-Span-2\" class=\"mrow\"><span id=\"MathJax-Span-3\" class=\"mi\">M<\/span><\/span><\/span><\/span>\u00a0into another\u00a0<span id=\"MathJax-Element-2-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-4\" class=\"math\"><span id=\"MathJax-Span-5\" class=\"mrow\"><span id=\"MathJax-Span-6\" class=\"mi\">N<\/span><\/span><\/span><\/span>\u00a0of negative curvature, there is a harmonic map. Furthermore, S. I. Al\u02b9ber [Dokl. Akad. Nauk SSSR\u00a0<span class=\"bf\">178<\/span>\u00a0(1968), 13\u201316;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=230254&amp;loc=fromrevtext\">MR0230254<\/a>] and P. Hartman [Canad. J. Math.\u00a0<span class=\"bf\">19<\/span>\u00a0(1967), 673\u2013687;<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=214004&amp;loc=fromrevtext\">MR0214004<\/a>] have established certain uniqueness results if\u00a0<span id=\"MathJax-Element-3-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-7\" class=\"math\"><span id=\"MathJax-Span-8\" class=\"mrow\"><span id=\"MathJax-Span-9\" class=\"mi\">N<\/span><\/span><\/span><\/span>\u00a0has strictly negative curvature. Our technique was to follow gradient lines (of the tension field of the energy function\u00a0<span id=\"MathJax-Element-4-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-10\" class=\"math\"><span id=\"MathJax-Span-11\" class=\"mrow\"><span id=\"MathJax-Span-12\" class=\"mi\">E<\/span><\/span><\/span><\/span>) in a suitable space of maps, since limit points are harmonic maps; that involved a rather delicate and explicit study of the appropriate elliptic and parabolic systems. We went on [the reviewer and Sampson, Proc. U.S.-Japan Sem. Differential Geometry (Kyoto, 1965), pp. 22\u201333, Nippon Hyoronsha, Tokyo, 1966;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=216519&amp;loc=fromrevtext\">MR0216519<\/a>] to show that without any curvature restrictions, there exists in every homotopy class a polyharmonic map, where the degree of polyharmonicity depends on the dimension of\u00a0<span id=\"MathJax-Element-5-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-13\" class=\"math\"><span id=\"MathJax-Span-14\" class=\"mrow\"><span id=\"MathJax-Span-15\" class=\"mi\">M<\/span><\/span><\/span><\/span>. The proof in that case was based on Morse theory on Hilbertian manifolds of maps, and in particular on verification of the Palais-Smale condition (C) for the poly-energy function.<\/p>\n<p class=\"review\">In the paper under review (which is an announcement, with sketch of proofs. Let us hope that a full account will appear in the near future, as these ideas deserve) a new method of proof is described for the existence of harmonic maps. It involves verification of (C) for perturbations (e.g., adding\u00a0<span id=\"MathJax-Element-6-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-16\" class=\"math\"><span id=\"MathJax-Span-17\" class=\"mrow\"><span id=\"MathJax-Span-18\" class=\"mi\">p<\/span><\/span><\/span><\/span>th powers of the differential, or of the Laplacian) of the energy\u00a0<span id=\"MathJax-Element-7-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-19\" class=\"math\"><span id=\"MathJax-Span-20\" class=\"mrow\"><span id=\"MathJax-Span-21\" class=\"mi\">E<\/span><\/span><\/span><\/span>, in the context of Morse theory on Finsler manifolds of maps. (A special case of these perturbations was discovered, for the same purpose, by H. I. Eliasson [<span class=\"it\">Global analysis<\/span>\u00a0(Proc. Sympos. Pure Math., Vol. XVI, Berkeley, Calif., 1968), pp. 67\u201389, Amer. Math. Soc., Providence, R.I., 1970;<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=267605&amp;loc=fromrevtext\">MR0267605<\/a>].) The method displays how the curvature restriction on\u00a0<span id=\"MathJax-Element-8-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-22\" class=\"math\"><span id=\"MathJax-Span-23\" class=\"mrow\"><span id=\"MathJax-Span-24\" class=\"mi\">N<\/span><\/span><\/span><\/span>\u00a0forces convergence of critical points of the perturbed energies to critical points of the energy itself. In addition to establishing existence, the method provides unique results (in the spirit of the proof of Al\u02b9ber). It would be interesting to see whether these methods permit extension to the Plateau problem (where\u00a0<span id=\"MathJax-Element-9-Frame\" class=\"MathJax\"><span id=\"MathJax-Span-25\" class=\"math\"><span id=\"MathJax-Span-26\" class=\"mrow\"><span id=\"MathJax-Span-27\" class=\"mi\">M<\/span><\/span><\/span><\/span>\u00a0has a boundary and harmonic maps have prescribed image on the boundary).<\/p>\n<p><span class=\"ReviewedBy\">Reviewed by\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=61935\">J. Eells<\/a><\/span><\/p>\n<hr \/>\n<p><a href=\"https:\/\/mathscinet.ams.org\/mathscinet-getitem?mr=464332\"><strong>MR0464332<\/strong><\/a>\u00a0<span style=\"color: #000000\"><b>\u00a0<\/b><\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=175775\">Uhlenbeck, K.<\/a><br \/>\n<span class=\"title\">Generic properties of eigenfunctions.<\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?journalKey=amer_j_math\"><em>Amer. J. Math.<\/em><\/a>\u00a0<strong>98\u00a0<\/strong>(1976), no. 4, 1059\u20131078.<\/p>\n<p class=\"review\">Let\u00a0<span class=\"MathTeX\">$M^n$<\/span>\u00a0be a compact\u00a0<span class=\"MathTeX\">$n$<\/span>-manifold and let\u00a0<span class=\"MathTeX\">$L_b$<\/span>\u00a0be a family of self-adjoint elliptic operators on\u00a0<span class=\"MathTeX\">$M^n$<\/span>\u00a0with the parameter\u00a0<span class=\"MathTeX\">$b\\in U$<\/span>\u00a0an open subset of a Banach space\u00a0<span class=\"MathTeX\">$B$<\/span>. The author shows that under reasonable hypotheses the following properties are generic with respect to\u00a0<span class=\"MathTeX\">$B$<\/span>, i.e., for almost all\u00a0<span class=\"MathTeX\">$b\\in U$<\/span>, (a)\u00a0<span class=\"MathTeX\">$L_b$<\/span>\u00a0has one-dimensional eigenspaces; (b) zero is not a critical value of the eigenfunctions, restricted to the interior of the domain of the operator; (c) the eigenfunctions are Morse functions on the interior of\u00a0<span class=\"MathTeX\">$M$<\/span>; (d) if\u00a0<span class=\"MathTeX\">$\\partial M\\neq\\varnothing$<\/span>\u00a0and Dirichlet boundary conditions have been imposed, then the normal derivative of the eigenfunctions has zero as a regular value. The author gives several applications. For example, let\u00a0<span class=\"MathTeX\">$\\Delta_g$<\/span>\u00a0be the Laplace operator for a metric\u00a0<span class=\"MathTeX\">$g\\in\\scr M_k=$<\/span>{<span class=\"MathTeX\">$C^k$<\/span>-metrics on\u00a0<span class=\"MathTeX\">$M^n$<\/span>} for\u00a0<span class=\"MathTeX\">$k&gt;n+3$<\/span>. Then {<span class=\"MathTeX\">$g\\in\\scr M_k\\colon\\Delta_g$<\/span>satisfies (a), (b), (c) and (d) on nonconstant eigenfunctions} is residual in\u00a0<span class=\"MathTeX\">$\\scr M_k$<\/span>.<\/p>\n<p><span class=\"ReviewedBy\">Reviewed by\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=174490\">A. J. Tromba<\/a><\/span><\/p>\n<hr \/>\n<p><a href=\"https:\/\/mathscinet.ams.org\/mathscinet-getitem?mr=0604040\"><strong>MR0604040<\/strong><\/a>\u00a0<span style=\"color: #000000\"><b>\u00a0<\/b><\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=299077\">Sacks, J.<\/a>;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=175775\">Uhlenbeck, K.<\/a><br \/>\n<span class=\"title\">The existence of minimal immersions of\u00a0<span class=\"MathTeX\">$2$<\/span>-spheres.<\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?journalKey=ann_of_math_2\"><em>Ann. of Math. (2)<\/em><\/a>\u00a0<strong>113\u00a0<\/strong>(1981), no. 1, 1\u201324.<\/p>\n<p class=\"review\">Let\u00a0<span class=\"MathTeX\">$M_p$<\/span>\u00a0be a closed Riemann surface of genus\u00a0<span class=\"MathTeX\">$p\\geq 0$<\/span>\u00a0and\u00a0<span class=\"MathTeX\">$N$<\/span>\u00a0a compact Riemannian manifold. In this interesting paper, the authors establish the existence of harmonic maps in three cases: (i) If\u00a0<span class=\"MathTeX\">$\\pi_2(N)=0$<\/span>, every homotopy class of maps\u00a0<span class=\"MathTeX\">$M_p\\rightarrow N$<\/span>\u00a0contains a harmonic map of minimum energy\u2014this was established by L. Lemaire [J. Differential Geom.\u00a0<span class=\"bf\">13<\/span>(1978), no. 1, 51\u201378;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=520601&amp;loc=fromrevtext\">MR0520601<\/a>]; see also the article by R. Schoen and S. T. Yau [Ann. of Math. (2)\u00a0<span class=\"bf\">110<\/span>\u00a0(1979), no. 1, 127\u2013142;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=541332&amp;loc=fromrevtext\">MR0541332<\/a>]. (ii) If\u00a0<span class=\"MathTeX\">$\\pi_2(N)\\neq 0$<\/span>, then a generating set for\u00a0<span class=\"MathTeX\">$\\pi_2(N)$<\/span>\u00a0modulo the action of\u00a0<span class=\"MathTeX\">$\\pi_1(N)$<\/span>\u00a0can be represented by harmonic maps\u00a0<span class=\"MathTeX\">$M_0\\rightarrow N$<\/span>\u00a0of minimum energy\u2014such a map is automatically a conformal branched immersion of minimum area. (iii) If\u00a0<span class=\"MathTeX\">$N$<\/span>\u00a0has noncontractible universal covering space, there exists a nonconstant conformal minimal branched immersion\u00a0<span class=\"MathTeX\">$M_0\\rightarrow N$<\/span>\u00a0(which may be a saddle point for the energy). The method is to perturb the energy functional\u00a0<span class=\"MathTeX\">$E$<\/span>\u00a0to give a functional\u00a0<span class=\"MathTeX\">$E_\\alpha$<\/span>\u00a0(<span class=\"MathTeX\">$\\alpha\\geq 1,E_1=E+$<\/span>\u00a0constant) which satisfies the condition (C) of Palais and Smale and then to study the complicated convergence of critical maps of\u00a0<span class=\"MathTeX\">$E_\\alpha$<\/span>\u00a0as\u00a0<span class=\"MathTeX\">$\\alpha\\rightarrow 1$<\/span>. Note that a nonexistence theorem contrasting with (ii) was given by A. Futaki [Proc. Japan Acad. Ser. A Math. Sci.\u00a0<span class=\"bf\">56<\/span>\u00a0(1980), no. 6, 291\u2013293;<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=581474&amp;loc=fromrevtext\">MR0581474<\/a>].<\/p>\n<p><span class=\"ReviewedBy\">Reviewed by\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=184435\">John C. Wood<\/a><\/span><\/p>\n<hr \/>\n<p><a href=\"https:\/\/mathscinet.ams.org\/mathscinet-getitem?mr=0664498\"><strong>MR0664498<\/strong><\/a>\u00a0<span style=\"color: #000000\"><b>\u00a0<\/b><\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=156695\">Schoen, Richard<\/a>;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=175775\">Uhlenbeck, Karen<\/a><br \/>\n<span class=\"title\">A regularity theory for harmonic maps.<\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?journalKey=j_differential_geom\"><em>J. Differential Geom.<\/em><\/a>\u00a0<strong>17\u00a0<\/strong>(1982), no. 2, 307\u2013335.<\/p>\n<p class=\"review\">Let\u00a0<span class=\"MathTeX\">$M^n$<\/span>\u00a0and\u00a0<span class=\"MathTeX\">$N^k$<\/span>\u00a0be Riemannian manifolds, with\u00a0<span class=\"MathTeX\">$M$<\/span>\u00a0compact. Let\u00a0<span class=\"MathTeX\">$u\\colon M\\rightarrow N$<\/span>\u00a0be an\u00a0<span class=\"MathTeX\">$L_1^2$<\/span>-map minimizing the energy functional\u00a0<span class=\"MathTeX\">$E(u)=\\int\\langle du(x),du(x)\\rangle dV$<\/span>, possibly modified to include lower order terms. Theorem: If\u00a0<span class=\"MathTeX\">$u(M)$<\/span>is in a compact subset of\u00a0<span class=\"MathTeX\">$N$<\/span>\u00a0a.e., then\u00a0<span class=\"MathTeX\">$u$<\/span>\u00a0is smooth on\u00a0<span class=\"MathTeX\">$M-\\scr S_u$<\/span>\u00a0for a suitable closed set\u00a0<span class=\"MathTeX\">$\\scr S_u$<\/span>\u00a0of Hausdorff dimension\u00a0<span class=\"MathTeX\">$\\leq n-3$<\/span>. If\u00a0<span class=\"MathTeX\">$n=3$<\/span>, then\u00a0<span class=\"MathTeX\">$\\scr S_u$<\/span>\u00a0is discrete. In fact,\u00a0<span class=\"MathTeX\">$\\scr S_u=\\{a\\in M\\colon\\liminf_{l\\rightarrow 0}E_{D_l(a)}(n)\/l^{n-2}&gt;0\\}$<\/span>. (An important special case of that result was obtained simultaneously by M. Giaquinta and E. Giusti [<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=648066&amp;loc=fromrevtext\">MR0648066<\/a>\u00a0above; Analysis, to appear]; there\u00a0<span class=\"MathTeX\">$u(M)$<\/span>\u00a0is required to lie in a coordinate chart.) The case\u00a0<span class=\"MathTeX\">$n=2$<\/span>\u00a0is due to C. B. Morrey; for arbitrary\u00a0<span class=\"MathTeX\">$n$<\/span>a key idea from potential theory is Morrey&#8217;s Dirichlet growth lemma [C. B. Morrey,\u00a0<span class=\"it\">Multiple integrals in the calculus of variations<\/span>, Springer, New York, 1966;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=202511&amp;loc=fromrevtext\">MR0202511<\/a>]. Another basic idea (used also by Giaquinta-Giusti), this time from geometric measure theory, is H. Federer&#8217;s reduction theorem [Bull. Amer. Math. Soc.\u00a0<span class=\"bf\">76<\/span>\u00a0(1970), 767\u2013771;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=260981&amp;loc=fromrevtext\">MR0260981<\/a>]. A major technical difficulty overcome in this paper is to find comparison maps (as used by Morrey) which have images in\u00a0<span class=\"MathTeX\">$N$<\/span>. By refining their arguments the authors obtain the theorem: In addition to the hypotheses of the preceding theorem, suppose that every harmonic map\u00a0<span class=\"MathTeX\">$\\theta\\colon S^j\\rightarrow N$<\/span>\u00a0is constant for\u00a0<span class=\"MathTeX\">$2\\leq j\\leq n-1$<\/span>, where\u00a0<span class=\"MathTeX\">$S^j$<\/span>\u00a0is the Euclidean\u00a0<span class=\"MathTeX\">$j$<\/span>-sphere; (more generally and importantly; if\u00a0<span class=\"MathTeX\">$p\\colon{\\bf R}^{j+1}-\\{0\\}\\rightarrow S^j$<\/span>\u00a0denotes radial projection and\u00a0<span class=\"MathTeX\">$\\theta\\colon S^j\\rightarrow N$<\/span>\u00a0is a harmonic map such that\u00a0<span class=\"MathTeX\">$\\theta\\circ p$<\/span>\u00a0minimizes energy on compact subsets of\u00a0<span class=\"MathTeX\">${\\bf R}^{j+1}$<\/span>, then\u00a0<span class=\"MathTeX\">$\\theta\\circ p$<\/span>\u00a0is constant\u00a0<span class=\"MathTeX\">$(2\\leq j\\leq n-1)$<\/span>). Then\u00a0<span class=\"MathTeX\">$\\scr S_u=\\varnothing$<\/span>; i.e.,\u00a0<span class=\"MathTeX\">$u$<\/span>\u00a0is smooth on\u00a0<span class=\"MathTeX\">$M$<\/span>. This is a very important contribution to the theory of harmonic maps. For instance, these hypotheses are satisfied if the universal cover of\u00a0<span class=\"MathTeX\">$N$<\/span>\u00a0supports a strictly convex smooth function. That case provides a new proof and generalization of the existence theorem of the reviewer and J. H. Sampson [Amer. J. Math.\u00a0<span class=\"bf\">86<\/span>\u00a0(1964), 109\u2013160;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=164306&amp;loc=fromrevtext\">MR0164306<\/a>] \u2014and much else, as well.<\/p>\n<p class=\"review\">{In the correction, it is noted that &#8220;harmonic&#8221; should be deleted from the statement of Lemma 2.5. The other results are not affected by the change.}<\/p>\n<p><span class=\"ReviewedBy\">Reviewed by\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=61935\">J. Eells<\/a><\/span><\/p>\n<hr \/>\n<p><a href=\"https:\/\/mathscinet.ams.org\/mathscinet-getitem?mr=0710054\"><strong>MR0710054<\/strong><\/a>\u00a0<span style=\"color: #000000\"><b>\u00a0<\/b><\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=156695\">Schoen, Richard<\/a>;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=175775\">Uhlenbeck, Karen<\/a><br \/>\n<span class=\"title\">Boundary regularity and the Dirichlet problem for harmonic maps.<\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?id=2374\"><em>J. Differential Geom.<\/em><\/a>\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publications.html?pg1=ISSI&amp;s1=8474\">18\u00a0<\/a><a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publications.html?pg1=ISSI&amp;s1=8474\">(1983),\u00a0<\/a><a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publications.html?pg1=ISSI&amp;s1=8474\">no. 2,<\/a>\u00a0253\u2013268.<\/p>\n<p class=\"review\">This paper follows an earlier one by the authors [same journal 17 (1982), no. 2, 307\u2013335;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=664498&amp;loc=fromrevtext\">MR0664498<\/a>]. Both papers are based on the facts that a harmonic map\u00a0<span class=\"MathTeX\">$u\\;(u\\in L^2_{1\\,{\\rm loc}}({\\bf R}^n,N))$<\/span>\u00a0which is constant along the rays from 0 a.e. defines a new map\u00a0<span class=\"MathTeX\">$w:S^{n-1} \\rightarrow N$<\/span>\u00a0such that\u00a0<span class=\"MathTeX\">$u(x)=w(x\/\\vert x\\vert )$<\/span>\u00a0which is also harmonic and conversely; furthermore <span class=\"MathTeX\">$u$\u00a0<\/span>has a singularity at 0 if and only if\u00a0<span class=\"MathTeX\">$w$<\/span>\u00a0is not a constant map (Theorems III and IV). This result supplies an estimate of the Hausdorff dimension of the set of singularities. The first paper dealt with inner regularity for harmonic maps, this one deals with boundary regularity for harmonic maps satisfying a Dirichlet problem. The boundary regularity is actually stronger. This is due to the fact that there are no nontrivial smooth harmonic maps from hemispheres\u00a0<span class=\"MathTeX\">$S^{n-j}_+$<\/span>\u00a0which map the boundary\u00a0<span class=\"MathTeX\">$S^{n-j-1}$<\/span>to a point\u00a0<span class=\"MathTeX\">$(1\\leq j\\leq n-2)$<\/span>. In fact, the Euler-Lagrange equation is deduced by minimizing a functional\u00a0<span class=\"MathTeX\">$\\tilde E$<\/span>\u00a0slightly more general than the energy\u00a0<span class=\"MathTeX\">$E,\\tilde E(u)=E(u) +V(u)$<\/span>, where\u00a0<span class=\"MathTeX\">$V(u)$<\/span>\u00a0is the integral over\u00a0<span class=\"MathTeX\">$M$<\/span>\u00a0of\u00a0<span class=\"MathTeX\">$\\Sigma_i\\Sigma_\\alpha\\,\\gamma ^\\alpha_i(x,u(x))\\partial u^i\/\\partial x^\\alpha+\\Gamma(x,u(x))$<\/span>. The main regularity theorem is the following: Let\u00a0<span class=\"MathTeX\">$M$<\/span>\u00a0be a compact manifold with\u00a0<span class=\"MathTeX\">$C^{2,\\alpha}$<\/span>\u00a0boundary. Suppose\u00a0<span class=\"MathTeX\">$u\\in L^2_1(M,N)$<\/span>\u00a0is\u00a0<span class=\"MathTeX\">$E$<\/span>-minimizing and satisfies\u00a0<span class=\"MathTeX\">$u(x)\\in N_0$<\/span>\u00a0a.e. for a compact subset\u00a0<span class=\"MathTeX\">$N_0\\subset N$<\/span>. Suppose\u00a0<span class=\"MathTeX\">$v\\in C^{2,\\alpha}(\\partial M,N_0)$<\/span>\u00a0and\u00a0<span class=\"MathTeX\">$u=v$<\/span>\u00a0on\u00a0<span class=\"MathTeX\">$\\partial M$<\/span>. Then the singular set\u00a0<span class=\"MathTeX\">$S$<\/span>\u00a0of\u00a0<span class=\"MathTeX\">$u$<\/span>\u00a0is a compact subset of the interior of\u00a0<span class=\"MathTeX\">$M$<\/span>; in particular,\u00a0<span class=\"MathTeX\">$u$<\/span>\u00a0is\u00a0<span class=\"MathTeX\">$C^{2,\\alpha}$<\/span>\u00a0in a full neighborhood of\u00a0<span class=\"MathTeX\">$\\partial M$<\/span>.<\/p>\n<p class=\"review\">An application is an amusing proof of a theorem of Sacks and Uhlenbeck on the existence of minimal 2-spheres representing the second homotopy group of a manifold: &#8220;If\u00a0<span class=\"MathTeX\">$N$<\/span>\u00a0is compact with convex or empty boundary, any smooth map\u00a0<span class=\"MathTeX\">$v:S^2\\rightarrow N$<\/span>which does not extend continuously to\u00a0<span class=\"MathTeX\">$B^3$<\/span>\u00a0is homotopic to a sum of smooth harmonic (hence minimal) maps<span class=\"MathTeX\">$u_j:S^2\\rightarrow N$<\/span>,\u00a0<span class=\"MathTeX\">$j=1,2,\\cdots,p$<\/span>&#8221;. The last section deals with approximation of\u00a0<span class=\"MathTeX\">$L^2_1$<\/span>\u00a0maps by smooth maps. The authors give a simple example of a map\u00a0<span class=\"MathTeX\">$u\\in L^2_1(B^3_1,S^2)$<\/span>\u00a0such that\u00a0<span class=\"MathTeX\">$u(x)=x\/\\vert x\\vert $<\/span>\u00a0cannot be an\u00a0<span class=\"MathTeX\">$L^2_1$<\/span>\u00a0limit of continuous maps; this result is still true for\u00a0<span class=\"MathTeX\">$L^2_1(M,N)$<\/span>\u00a0with\u00a0<span class=\"MathTeX\">$\\dim M\\geq 3$<\/span>. On the contrary if\u00a0<span class=\"MathTeX\">$\\dim M=2$<\/span>, an\u00a0<span class=\"MathTeX\">$L^2_1$<\/span>\u00a0map is a limit of smooth maps.<\/p>\n<p><span class=\"ReviewedBy\">Reviewed by\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=40115\">Liane Valere Bouche<\/a><\/span><\/p>\n<hr \/>\n<p><a href=\"https:\/\/mathscinet.ams.org\/mathscinet-getitem?mr=861491\"><strong>MR0861491<\/strong><\/a>\u00a0<span style=\"color: #000000\"><b>\u00a0<\/b><\/span><br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=175775\">Uhlenbeck, K.<\/a><span class=\"instInfo\"><a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/institution.html?code=1-CHI\">(1-CHI)<\/a><\/span>;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=185480\">Yau, S.-T.<\/a><br \/>\n<span class=\"title\">On the existence of Hermitian-Yang-Mills connections in stable vector bundles.<\/span><br \/>\nFrontiers of the mathematical sciences: 1985 (New York, 1985).<br \/>\n<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/journaldoc.html?id=2342\"><em>Comm. Pure Appl. Math.<\/em><\/a>\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publications.html?pg1=ISSI&amp;s1=7173\">39\u00a0<\/a><a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publications.html?pg1=ISSI&amp;s1=7173\">(1986),\u00a0<\/a><a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publications.html?pg1=ISSI&amp;s1=7173\">no. S, suppl.,<\/a>\u00a0<span class=\"rm\">S<\/span>257\u2013<span class=\"rm\">S<\/span>293.<\/p>\n<p class=\"review\">The Yang-Mills equations, which arise in particle physics, have been applied with great success to study the differential topology of\u00a0<span class=\"MathTeX\">$4$<\/span>-manifolds. Of particular interest are complex surfaces, in which case the Yang-Mills equations have a holomorphic interpretation. Let\u00a0<span class=\"MathTeX\">$X$<\/span>\u00a0be a compact K\u00e4hler manifold (of any dimension) and\u00a0<span class=\"MathTeX\">$E$<\/span>\u00a0a holomorphic bundle over\u00a0<span class=\"MathTeX\">$X$<\/span>. A Hermitian metric on\u00a0<span class=\"MathTeX\">$E$<\/span>\u00a0determines a canonical unitary connection whose curvature is a\u00a0<span class=\"MathTeX\">$(1,1)$<\/span>-form\u00a0<span class=\"MathTeX\">$F$<\/span>\u00a0which is a skew-Hermitian transformation of\u00a0<span class=\"MathTeX\">$E$<\/span>. The inner product of\u00a0<span class=\"MathTeX\">$F$<\/span>\u00a0with the K\u00e4hler form is then an endomorphism of\u00a0<span class=\"MathTeX\">$E$<\/span>, and the connection satisfies the Yang-Mills equations if and only if this endomorphism is a multiple of the identity. Metrics which give rise to such connections are called Hermitian-Einstein metrics, and the resulting connection is termed Hermitian-Yang-Mills. It is natural to ask which bundles admit such metrics. Recall first a definition from algebraic geometry. The slope of a bundle is the ratio of its degree to its rank, and a bundle\u00a0<span class=\"MathTeX\">$E$<\/span>\u00a0is said to be stable if the slope of any coherent subsheaf of lower rank is strictly less than the slope of\u00a0<span class=\"MathTeX\">$E$<\/span>. The main theorem of the present paper asserts that a stable holomorphic bundle over a compact K\u00e4hler manifold admits a unique Hermitian-Einstein metric. For complex curves this is an old result of Narasimhan and Seshadri. S. K. Donaldson [Proc. London Math. Soc. (3)\u00a0<span class=\"bf\">50<\/span>\u00a0(1985), no. 1, 1\u201326;\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=765366&amp;loc=fromrevtext\">MR0765366<\/a>] gave a proof for projective algebraic surfaces, and (subsequent to the work of the authors) extended his work to cover projective complex manifolds of any dimension [Duke Math. J.\u00a0<span class=\"bf\">54<\/span>\u00a0(1987), no. 1, 231\u2013247;<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/publdoc.html?r=1&amp;pg1=MR&amp;s1=885784&amp;loc=fromrevtext\">MR0885784<\/a>].<\/p>\n<p class=\"review\">The authors make a direct study of the partial differential equation arising from the Hermitian-Yang-Mills condition. The continuity method is used to demonstrate the existence of solutions to a perturbed equation. Of course, this involves a priori estimates for the solutions. Let\u00a0<span class=\"MathTeX\">$\\varepsilon$<\/span>\u00a0be the perturbation parameter, and\u00a0<span class=\"MathTeX\">$h_\\varepsilon$<\/span>\u00a0the solution to the perturbed equation. Then as\u00a0<span class=\"MathTeX\">$\\varepsilon\\rightarrow 0$<\/span>\u00a0either the\u00a0<span class=\"MathTeX\">$h_\\varepsilon$<\/span>\u00a0converge to a solution, or (after appropriate renormalization) the limit represents an\u00a0<span class=\"MathTeX\">$L_1^2$<\/span>\u00a0holomorphic projection onto a subbundle. A major step in the proof consists in showing that this projection is smooth outside a subvariety of codimension at least\u00a0<span class=\"MathTeX\">$2$<\/span>\u00a0and that the image is a coherent subsheaf of\u00a0<span class=\"MathTeX\">$E$<\/span>. Then the previous estimates are used, via the Chern-Weil theory, to show that this subsheaf is destabilizing for\u00a0<span class=\"MathTeX\">$E$<\/span>.<\/p>\n<p class=\"review\">The regularity theorem for the projection is stated in general terms. Let\u00a0<span class=\"MathTeX\">$M$<\/span>\u00a0be an algebraic manifold, which we assume is embedded in some projective space. A map\u00a0<span class=\"MathTeX\">$F$<\/span>\u00a0from the unit ball in\u00a0<span class=\"MathTeX\">$\\mathbf{C}$<\/span>\u00a0to\u00a0<span class=\"MathTeX\">$M$<\/span>\u00a0is said to be weakly holomorphic if it is in\u00a0<span class=\"MathTeX\">$L_1^2$<\/span>\u00a0and if its differential maps the holomorphic tangent space of the ball into the holomorphic tangent space of\u00a0<span class=\"MathTeX\">$M$<\/span>\u00a0almost everywhere. For balls of higher dimension a map is weakly holomorphic if for every linear coordinate system\u00a0<span class=\"MathTeX\">$\\{z_1,\\cdots,z_n\\}$\u00a0<\/span>and for almost every value of\u00a0<span class=\"MathTeX\">$z_2,\\cdots,z_n$<\/span>\u00a0it is weakly holomorphic as a function of\u00a0<span class=\"MathTeX\">$z_1$<\/span>. Then the authors prove that any weakly holomorphic map into an algebraic manifold is meromorphic.<\/p>\n<p class=\"review\">Both the result and the techniques of this paper are important. They have already found use in the theses of K. Corlette [&#8220;Flat\u00a0<span class=\"MathTeX\">$G$<\/span>-bundles with canonical metrics&#8221;, J. Differential Geom., to appear] and C. Simpson [&#8220;Systems of Hodge bundles and uniformization&#8221;, Ph.D. Thesis, Harvard Univ., Cambridge, Mass., 1987; per revr.].<\/p>\n<p><span class=\"ReviewedBy\">Reviewed by\u00a0<a href=\"https:\/\/mathscinet.ams.org\/mathscinet\/search\/author.html?mrauthid=69015\">Daniel S. Freed<\/a><\/span><\/p>\n<div style=\"margin-top: 0px; margin-bottom: 0px;\" class=\"sharethis-inline-share-buttons\" ><\/div>","protected":false},"excerpt":{"rendered":"<p>Karen Uhlenbeck is being awarded the 2019 Abel Prize.\u00a0 It is a remarkable award for a remarkable mathematician.\u00a0 Uhlenbeck did fundamental work in a quickly developing area of mathematics at an early stage of its development.\u00a0 I was a graduate &hellip; <a href=\"https:\/\/blogs.ams.org\/beyondreviews\/2019\/03\/25\/karen-uhlenbeck-wins-the-2019-abel-prize\/\">Continue reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n<div style=\"margin-top: 0px; margin-bottom: 0px;\" class=\"sharethis-inline-share-buttons\" data-url=https:\/\/blogs.ams.org\/beyondreviews\/2019\/03\/25\/karen-uhlenbeck-wins-the-2019-abel-prize\/><\/div>\n","protected":false},"author":86,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[35],"tags":[],"class_list":["post-2400","post","type-post","status-publish","format-standard","hentry","category-mathematics-in-the-news"],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p6C2KK-CI","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/posts\/2400","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/users\/86"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/comments?post=2400"}],"version-history":[{"count":7,"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/posts\/2400\/revisions"}],"predecessor-version":[{"id":2408,"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/posts\/2400\/revisions\/2408"}],"wp:attachment":[{"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/media?parent=2400"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/categories?post=2400"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.ams.org\/beyondreviews\/wp-json\/wp\/v2\/tags?post=2400"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}