CINXE.COM

<!DOCTYPE html><html lang="en-US"><head><title>Just a moment...</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=Edge"><meta name="robots" content="noindex,nofollow"><meta name="viewport" content="width=device-width,initial-scale=1"><style>*{box-sizing:border-box;margin:0;padding:0}html{line-height:1.15;-webkit-text-size-adjust:100%;color:#313131;font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji}body{display:flex;flex-direction:column;height:100vh;min-height:100vh}.main-content{margin:8rem auto;max-width:60rem;padding-left:1.5rem}@media (width <= 720px){.main-content{margin-top:4rem}}.h2{font-size:1.5rem;font-weight:500;line-height:2.25rem}@media (width <= 720px){.h2{font-size:1.25rem;line-height:1.5rem}}#challenge-error-text{background-image:url(data:image/svg+xml;base64,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);background-repeat:no-repeat;background-size:contain;padding-left:34px}@media (prefers-color-scheme:dark){body{background-color:#222;color:#d9d9d9}}</style><meta http-equiv="refresh" content="390"></head><body class="no-js"><div class="main-wrapper" role="main"><div class="main-content"><noscript><div class="h2"><span id="challenge-error-text">Enable JavaScript and cookies to continue</span></div></noscript></div></div><script>(function(){window._cf_chl_opt={cvId: '3',cZone: "direct.mit.edu",cType: 'managed',cRay: '92724071abdcd4de',cH: 'K7IC1MHd9NEbfaaFW3veRJC40JciBFJgc86lKhg4LZE-1743113782-1.2.1.1-6UfwCrqt7MijlTOm06KKlYeN2QjmjPdSXaxpHA.9hFwVDjj3h0AF.dTLA1UYt5T9',cUPMDTk: "\/artm\/article\/6\/2\/50-71\/18031?__cf_chl_tk=jAZRmZ9b26Voz4iFHybc9y1WwEQAoAU9NOo32B03IYk-1743113782-1.0.1.1-VaQCvCdbcjMe2nLXhSAKPN1S_ayFp40vWn3GWJyoPzo",cFPWv: 'b',cITimeS: '1743113782',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/artm\/article\/6\/2\/50-71\/18031?__cf_chl_f_tk=jAZRmZ9b26Voz4iFHybc9y1WwEQAoAU9NOo32B03IYk-1743113782-1.0.1.1-VaQCvCdbcjMe2nLXhSAKPN1S_ayFp40vWn3GWJyoPzo",md: "ANWQ4i4qopxxbqGlZWscDarO.lY_lLNJ352HDu0kRl8-1743113782-1.2.1.1-C3LcjKJiPnqoxvXayYZ18yfobSzvpFBuxeJWYd55BnlDDJY5YetwhMLQgLJ2rqX5kABtuWyeKIcmsSM_fK0ksmy3z4I51XdLVjED5OeI8ZsKOuvadX1W4L6sm_2_SBiSCqL97czH_MsnU2k7D7x3HMP8jbj_Ox09xzdh9FJO4F4JUSxMlUj5u8pLNWOOxDqe0cbuXUNYqNqXpXLREDCjl1F0S2YrjUonts4Mgtsf3ME6jWff5_s0VphxFpOa1cg7pH4LcSCY7G4H4jpFA_GlqPvC6JzCvZ6ZVXHDs8czXCscnJulkiHLE.U0ESL2JCJvJ6B553nlPrFtgF6fsoTOqQwxowelhXlS5tNfW_ULwwskZUgG3.0QThgEb4Em7AcoX9Hh9Vqh6IgvORn0yrl.CfxUlFaNXXiW5_Il16UM58cDvdNoBegT1iaNNh0awR4nuAhsvdx5.g9MS3Qk0hRm.QndFrf3VfNVt8QBQ.0NEaIcyDxV_6qj9gO0rpJjUBxFWDVnL1EksXA8lSQqU2Yp7akslIdL6sdmwZcX7dhVVPEW7B8daTE6DbJRHRbwbDvLCs572HAHmBvG.PWkrS.0Eigd0mL915xCanD_o9ZNH6WMFS29NnAU6zHCSouY5WKT0EVMvL9yy2H5e.Vrc3jqaU.xGJZGRiyfB5.3_AfRgYRTrM_yLKCzFWJ9psJm8wW3R3UTYk61rnYW1S3LH1e8K8DDcaXnO8pjd_OCOATA724utRJKOPTxUXxydRXrNrB0XGbspccs8NKhFbJZmeMCIUV0AlnpEyjAjPSaxcwXAjqyvVSk6RLiDfdFHvlcW6C7xqeSJYjAzW5DY65Whrlg_AM4D_rSfudlgNtgEv4zXM2iJg_96d4Wkf8uGcBWdtJLi2Ekd5eScsUBBwpHNn_IQ.3MRdR54Nl0kMQuVraUSmEd8boSjvurW0TkcAdr3TdLnrT7YEQF1Iut3XYtCVrnjZmHEhv.Vfbl4KCGaC0pjHWbbV3nqVlwZiXurlbZaSYnyNk_xUBJ8JS6KL0DvLwfeXTYvdIVc78C7M1zMx4T.bgoN50aElt9KNNdBTRA1sUIF7jgR9WW_akZ4A.Ih_2jRmoYSwpZ6OftyriH6a1FWzN9W4_4rB0D7uutwDOCPHjCrdfE8Hmis21fb8GcLBykCAfKlpZzkydDAG4TVMtMaENjBmEjZzpbTwr27ku6osGorOI1NpSBaeNoq2l8OOp3Xezbd5WJ7VpxMP0PSAkBL1ayvue2AoGkBZ55N_8d_rXnMrfCq9Oc9wENp_0yEvQEwQ",mdrd: "h4_X5Vuupdw9epYomFadNcxUHQGmc50C4Q.Jz0bGLAc-1743113782-1.2.1.1-yLB6GqHOGkJPoudg2.wRU7XtZ3hRGVe1qTENx9hk_XlwJlNN0He1EEJ33.7Rye4daw5c4WlY8d.9XtDu9IZ4g3l0HWuXoIzlUFTkr3cvtTiqYysdJkxzKMIoJrFwLvXuy6nTel_oPC8SikOocB0GqPJchvVtuQNzn1XzR_C5ztJtdqZp0l6PXDvYrAHdOj1R8lSr5I0OOROEhjh.zD1yJcez0TMRczh4pQZZt0mAenkDXzePwdQ5HY8Gu_uZeuGf4wlXhFNYl_JC.YTI0C8Jh3JkNjTAxPnYHoHf_eiLudzJjcEXX4JJbwG_6Ii.NJ4BXC.NsPLBCpXozWMwfkWZmaZB4N2nu5MEDbZp0fvwSNRMbKd8XACLLAy_vXWAv6KqpobmAQGGEfWwh49tBj5qOl97pybsktV7710G09PClEf7MnUPTvoy0OqJ2SYkAPbJxeEepF57IsaBvtpXEVPWupZDJQ.pfEOS3cx2KApmshEhtgWRjg5McWb1782p_H2myJgjITemev4xXIXyteuj2QodehYXMkKq1..IN_mrZGd7zbgWKkMMHBznuMtss5D6jZBG8Bsyi75LrCfO75B1fj2rWAT3gYo4DCO4nGf_Y8vdSFWS9UTL1vk3rwR6W49qBzFj3m8TcFPKqoCKoGf2SxsOHtGbxbUBcmVeI739yz.Qn1LKsux78vKWmjn3R9NVVtnb806X3wiEPT1DbB.4u5aaFdPUOQpzRw_i9agzEbAgV49KOH8uUieGrdFyvPaqtWGX5.YVCMwniIrqUv.kqrPi1APUreO7F_jyEYWmukdLjVRqLKZsdTJm5gNjc9xfOKYOLK.g4PllylRqDUa8rGkrW8WawSR_Ur8tZA2LSOe1abbhvTjTD_qor6I_anTZb7Rul8OZNEdSHhes8mjW2J34ZPkXhlURPW07oUFWnCyjiFYBHcfWUJbdCoELAawbj4CmS5cEO5DzX3vIc4yExwhEUlWUj0GVT7GUBlIUa7AECUVDm2PKRY8KcYwwsckwlL0qiqgcA1fzIN9P9n5NDVeKDIcPh37mLGOKSB3qOBoCtbwF9tWTE5WUqMikLIxn_fbjNGm7tPG5DZaGYI2Ccw4s1VHwGJWyBw1UNcvDOA1bciILJi20JRm7YqWN3MsvbrsmuYlmQf0VCV1niq3OpOBH_ayYv0NA57Lre7zVIGrdvJ2ttD1WFWIt0luWHjuPRKLHuScIPQq3J_z3Us7ozPIOXinqLk2LNgry9R3QUSinNPeiQfb9aMMH_9PlHJVEWYObYwCdSJX.dGzstAj51l0i1.PwFWF_.BoEUyAd.JEdmrJe0fCsjxcNSq2OV.abY18nVo_rAI5Wc44xf_HCD1FmNyHhOyU3fJGWSaAndM__hMSeYGQ9VY7jmEl5NkC6X8T5xQPF7FRZII19YMMgCeWELXhmysEoSgrN7xxfaWhpkvMmbwVdYSC.9IPJZCZTe0Ovj_NVpJhyoJwvzP9Mtv7le68NEsNavSyEgxWRuJrsRD6Pdt17GLPsUwm.tgmWokKEQlh8RbtCcdoYGvl.MiMbme.yFWgFhYtXhY9aUmPE1fAy6hnSm8_at_F3KIMd4TtSHAgSfhY_V.JRglsTZrbKcJ7dkzfhxinuNA6b1y2jjExH0L29xlqRsz3CS9OfKqd9IgqonZ93AdXzSA0ROWRoHkx2Vl9xH1jHMoRWoeTgjzSsI8ntcDbOdicyAndvUoo92SYaH20kftosET5sj5JNwlRPHwvUidyyQXLPq_vk5ZJ4XY4kLL_ymmFOQFhhzDQtyOP35XgqAZVroZtLwFK1m4XhGacVIn1bazOLYlJ1yZODbRtEuXNps5wrul4I3th9Z7sQMjnC0C1aiARy3R7y1VzW69CDbpplPKqbeg2B_TmP2w7iKCaX.6ksK.mZ0cmG1F9q3Qr14U05aBT1lQmuNmXnW9HOzR5g9C.Sx.hpk1GO7HeBhoWt5nk2nB2KjrVQpaP2M6j8uClyXtj.0AWHI_uP1Bkq90g.M4ydRyZJIX_fFsRohg97mU_.VdY73HGknHeG8Wsv9Xq2BFXpeg"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=92724071abdcd4de';window._cf_chl_opt.cOgUHash = location.hash === '' && location.href.indexOf('#') !== -1 ? '#' : location.hash;window._cf_chl_opt.cOgUQuery = location.search === '' && location.href.slice(0, location.href.length - window._cf_chl_opt.cOgUHash.length).indexOf('?') !== -1 ? '?' : location.search;if (window.history && window.history.replaceState) {var ogU = location.pathname + window._cf_chl_opt.cOgUQuery + window._cf_chl_opt.cOgUHash;history.replaceState(null, null, "\/artm\/article\/6\/2\/50-71\/18031?__cf_chl_rt_tk=jAZRmZ9b26Voz4iFHybc9y1WwEQAoAU9NOo32B03IYk-1743113782-1.0.1.1-VaQCvCdbcjMe2nLXhSAKPN1S_ayFp40vWn3GWJyoPzo" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>