PROFILE

Profile of Christos PapadimitriouFarooq AhmedScience Writer

When Christos Papadimitriou graduated in1972 from the National Technical Universityof Athens in his native Greece, he arrived ata breaking point. Like all Greek men he wasrequired to join the armed forces, and heenlisted while the country struggled un-der military rule.“I went in fully expecting to serve and then

work quietly as an engineer somewhere,”Papadimitriou recalls. “But during militaryservice, the ruthlessness of the regime struckme in a very powerful way. I knew that Icould not stay.” He applied to graduateschool and joined Princeton University’sfledgling computer science group. Demo-cratic reforms in 1974 ousted Greece’s mili-tary junta the same year that Papadimitriouearned a Master’s degree. He would com-plete his doctorate two years later in elec-trical engineering and computer science.In the past four decades, Papadimitriou’s

influence on the field of computer science hasbeen far-reaching. His articles launched newresearch areas, his textbooks have becomestandards, and his works of fiction—two nov-els and a graphic novel—have introduced thehistory of logic and computer science tobroad audiences. In 1996 Papadimitrioujoined the University of California, Berkeley,where he holds the C. Lester Hogan Chairof Electrical Engineering and Computer Sci-ence. He was elected to the National Acad-emy of Sciences in 2009.

Leaving AthensBorn to educators in a small town outside ofDelphi in theGreekmountains, Papadimitrioumoved to Athens with his family when hewas five years old. The “mountain accent” hecarried into the financial and political capi-tal of Greece earned him few friends. “Thatgrief led me to studying as a way of escapingmy social problems.”At the National Technical University of

Athens, Papadimitriou studied mechani-cal and electrical engineering. However,he wasn’t keen on the subject. Instead,Papadimitriou stole into the university’scomputer laboratory and taught himselfhow to program. “There was an IBM 1620computer, probably a million times lesspowerful than today’s cellphones. It was

closely guarded but that was the onlything that really got my attention.”

ComplexityAfter graduating from college and servingin the military, Papadimitriou left Greecefor northern New Jersey. When he arrivedat Princeton in 1973, the computer sciencegroup at the university had five people.However, for Papadimitriou the settingprovided a stimulating intellectual environ-ment and an opportunity to influence anemerging field.“For the first time in my life,” he admits, “I

found something that I really enjoyed—and Iwas good at it! Suddenly, I decided to takemy graduate studies very seriously.” Papadi-mitriou worked with Kenneth Steiglitz andbegan his lifelong investigations into com-putational complexity.“It all started with Alan Turing,” Papadi-

mitriou says. In 1936 Turing, a Britishlogician who established the conceptualframework for computer science, provedthat some problems could not be solvedby computers: that no algorithm could bewritten to find solutions, even if the questionswere well-defined (1). “Computer sciencewas a field born with a dire understandingof its limitations,” adds Papadimitriou.Building on Turing’s and others’ work, in

1971 the University of Toronto’s StephenCook devised a way to distinguish betweenproblems that could be solved quickly, inpolynomial time, and others for which thetime to uncover a solution increased ex-ponentially (2). In the field’s parlance, thelatter class of problems is now referred toas “NP-complete” or “NP-hard.”Papadimitriou describes the difference

between these types of problems as similarto trying to find a needle in a haystack. “Ifthe needle’s ferromagnetic then you can usea magnet, and you’re done. But what if thatneedle is plastic? You have to look underevery blade of hay.”“I decided to dedicate my career to try and

understand the difference between the nee-dles,” Papadimitriou says. “That’s complexitytheory.” Papadimitriou proved that the his-toric traveling salesman problem, in whichone has to calculate the shortest route

between several cities such that each cityis only visited once, is of the NP-complete(3) type. His work on complexity, oftenwith collaborator Mihalis Yannakakis, nowat Columbia University, transformed andexpanded the subject (4).

Academic FreedomWhen he received his doctorate fromPrinceton, computer science was still a smallenough field that Papadimitriou felt like he“knew all that there was to know at thetime.” He left as a theoretician with a de-termination to advance the field and ensurethat it became one that would solve “sig-nificant and meaningful” problems.Over the next two decades, Papadimitriou

taught at six different institutions: HarvardUniversity, Massachusetts Institute of Tech-nology, Stanford University, the NationalTechnical University of Athens, the Uni-versity of California at San Diego, andthe University of California at Berkeley.Papadimitriou’s return in 1981 to Athens,where he taught for at least one semesterfor the next seven years, provided, he says,a vindication. “It was a bold, perhaps risky

Christos Papadimitriou. Image courtesy ofEirene-Chloe Markenscoff Papadimitriou.

This is a Profile of a recently elected member of the National Academy

of Sciences to accompany the member’s Inaugural Article, 10.1073/

pnas.1416954111.

www.pnas.org/cgi/doi/10.1073/pnas.1405579111 PNAS Early Edition | 1 of 3

PROFILE

decision for me to return. But I was excitedto contribute to the development of com-puter science in Greece.”It was at Berkeley’s northern California

campus, however, that Papadimitriou foundan intellectual home. “There is a very ag-gressive form of academic freedom at Ber-keley. I first visited as a Miller Fellow in 1978,and it gave me a feeling of extreme freedom:If it feels good intellectually, then you mustdo it! And do it with passion.”From the mid-1970s to the mid-1990s,

as computer science matured, Papadimitrioutaught nearly every course, from program-ming languages to compilers, operatingsystems, and databases. His 1982 textbook,Elements of the Theory of Computation (5),written with Harvard University’s HarryLewis, explained the foundations of emerg-ing fields and influenced how courses on thesubject were taught. With his graduateadviser Steiglitz, Papadimitriou publisheda graduate textbook on algorithms andcomplexity called Combinatorial Optimi-zation in 1982 (6). A more recent text-book on algorithms has become a standardfor undergraduates (7).“Frankly,” Papadimitriou explains, “teach-

ing is the only way I know to understandsomething. But these days,” he adds with alaugh, “the courses have become so com-plicated that I doubt I could pass them!”In his research, Papadimitriou used mathe-

matical tools to uncover complexity, wher-ever it exists. He defined new classes ofcomplexity, which have led to breakthroughsand new ways of understanding computa-tional problems (8, 9). Extending his workbeyond computer science, Papadimitriou

and colleagues found that a two-dimensionalprotein-folding model, the hydrophobic-polar model, was of the NP-complete type(10). Papadimitriou’s work on databases inthe 1970s and 1980s culminated in a 1986monograph, The Theory of Database Con-currency Control (11).

The World Wide WebAfter returning to the University ofCalifornia, Berkeley in January 1996,Papadimitriou reached another inflectionpoint in his career. In the early 1990s, theInternet and the World Wide Web hadreached a critical moment. A plethora ofsmall voices were virtually publishing con-tent, and major companies were profitingwithout a brick-and-mortar presence.“The Internet,” explains Papadimitriou,

“was something completely new and dif-ferent. Like mathematics, its existence isbased on minimalistic principles, but youcannot point to a designer, a committee, auser, or a company that made it happen. Itgrew out of the interaction of many.”This line of reasoning led to a realization:

“Computer science was no longer about thecomputer. The creation of the Internet wastransforming computer science into a nat-ural science. But it was also transformingcomputer science into a social science! Be-cause you cannot understand the Internetwithout understanding the incentives of theentities that use and continue to create it.”Papadimitriou focused on understand-

ing the Internet as a neuroscientist ap-proaches the brain or an astrophysicistviews the universe. For his contributions,in 2002 Papadimitriou received the Donald E.

Knuth Prize, awarded jointly by the Associ-ation for Computing Machinery and theInstitute of Electrical and ElectronicsEngineers.

AnarchyPapadimitriou’s work has helped create andengage new scientific disciplines. Investiga-tions into decision making and mathematicalmodels of conflict resolution drove contri-butions to algorithmic game theory, a fieldat the intersection of economics and com-puter science.In 1999 Papadimitriou and Elias Kout-

soupias, now at the University of Oxford,defined the price of anarchy, a mathematicalway of measuring how performance in asystem degrades as a result of the selfishnessof its users (12). “Imagine,” Papadimitriouexplains, “that the Internet was not run bythousands of providers but instead by onebenevolent dictator who wanted to maxi-mize social welfare. How much better wouldit be than what we have today? That’s whatwe call the price of anarchy.” Often, theanswer is, “Not that much.”His recent work in game theory concerns

the Nash equilibrium, a fundamental conceptin economics for which Princeton’s JohnNash, won a Nobel Prize in 1994. The Nashequilibrium describes the situation in which

Papadimitriou and his team discuss how to tell the story of British mathematician BetrandRussell’s epic search for the truth. Image courtesy of Apostolis Doxiadis, Alecos Papadatos,Annie di Donna, and Anne Bardy, 2009, Logicomix Print Ltd. and Bloomsbury Publishing Plc.

Christos as featured in LogiComix. Imagecourtesy of Apostolos Doxiadis, ChristosPapadimitriou, Alecos Papadatos, and Anniedi Donna, 2009, Logicomix Print Ltd. andBloomsbury Publishing Plc.

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agents in a market or at a game who knowthe behavior of their competitors do notchange their own strategies because doingso would not be gainful. As a result, theagents reach an unchangeable balance.Papadimitriou and colleagues determinedthat finding the Nash equilibrium in agame is an intractable problem (13). For hisresearch on algorithmic game theory, in 2012Papadimitriou received the Gödel Prizefor theoretical computer science, and heshared with his coauthors, Paul Goldbergand Constantinos Daskalakis, the GameTheory Society’s prize in Game Theory andComputer Science.

Algorithmic SexIn addition to teaching at Berkeley, Papadi-mitriou holds a senior scientist position at theuniversity’s Simons Institute for the Theoryof Computing, a post that has allowedhim to further leverage his understandingof computation. By studying algorithmsthat simulated biological reproduction, heand others explained a persistent mystery inevolutionary biology: Why is sexual repro-duction so prevalent in nature when asexualreproduction seems far more efficient atpromoting fitness (14, 15)?Papadimitriou explains that “sex does a

better job at increasing mixability, which isthe ability of different genes to perform invaried genetic environments. This givesorganisms better long-term chances.” Headds that “there are many more mysteries

remaining in evolution, and I have a feelingthat computation and complexity mayhold the key.” Papadimitriou’s InauguralArticle highlights the utility of compu-tation in areas beyond computer scienceand deploys computation as a metaphorto reveal approaches to challengingproblems in science (16).

StoriesIn the last decade and a half, in parallelwith his teaching and research, Papadi-mitriou has embraced the humanities. Hepublished two novels: Turing (A Novelabout Computation) (17), a love storythat features a virtual Alan Turing as

a character, and Independence (18), afamily drama that tackles the history ofmodern Greece. With author ApostolosDoxiadis, Papadimitriou wrote the in-ternationally best-selling graphic novelLogicomix (19), dramatizing Bertrand Rus-sell’s quest to codify the logical foundations ofmathematics.Papadimitriou views his novelistic works

as an extension of his science. “I startedwriting quite suddenly, and it made mevery aware of how important stories are inscience, in my science. I’ve found that I canalmost never understand a concept unlessI reach it through a story.”

1 Turing AM (1937) [Delivered to the Society November 1936]. Oncomputable numbers, with an application to the Entscheidungsproblem” P Lond Math Soc Series 2,42:230–265.2 Cook SA (1971) The complexity of theorem-proving procedures.Proceedings of the 3rd Annual ACM Symposium on Theory ofComputing (ACM, New York), pp 151–158.3 Papadimitriou CH (1977) The Euclidean travelling salesmanproblem is NP-complete. Theor Comput Sci 4(3):237–244.4 Papadimitriou CH, Yannakakis M (1991) Optimization, approximation,and complexity classes. J Comput Syst Sci 43(3):425–440.5 Lewis HR, Papadimitriou CH (1982) Elements of the Theory ofComputation (Prentice-Hall, Englewood Cliffs, NJ).6 Papadimitriou CH, Steiglitz K (1982) Combinatorial Optimization(Prentice-Hall, Englewood Cliffs, NJ).7 Dasgupta S, Papadimitriou CH, Vazirani U (2006) Algorithms(McGraw-Hill, New York).8 Johnson DS, Papadimitriou CH, Yannakakis M (1988) How easy islocal search? J Comput Syst Sci 37(1):79–100.9 Papadimitriou CH (1994) On the complexity of the parity argument andother inefficient proofs of existence. J Comput Syst Sci 48(3):498–532.10 Crescenzi P, Goldman D, Papadimitriou C, Piccolboni A,Yannakakis M (1998) On the complexity of protein folding.J Comput Biol 5(3):423–465.

11 Papadimitriou CH (1986) The Theory of Database Concurrency

Control (Computer Science Press, New York, NY).12 Koutsoupias E, Papadimitriou CH (2009) Worst-case equilibria.

Comput Sci Rev 3(2):65–69.13 Daskalakis C, Goldberg P, Papadimitriou CH (2006) The

complexity of computing a Nash equilibrium. Proceedings of

the Thirty-Eighth Annual ACM Symposium on the Theory

of Computing (ACM, New York), 71–78.14 Livnat A, Papadimitriou C, Dushoff J, Feldman MW (2008) A

mixability theory for the role of sex in evolution. Proc Natl Acad Sci

USA 105(50):19803–19808.15 Livnat A, Papadimitriou C, Pippenger N, Feldman MW (2010) Sex,

mixability, and modularity. Proc Natl Acad Sci USA 107(4):1452–1457.16 Papadimitriou CH (2014) Algorithms, complexity, and the

sciences. Proc Natl Acad Sci USA, 10.1073/pnas.1416954111.17 Papadimitriou CH (2003) Turing (A Novel about Computation)

(The MIT Press, Cambridge, MA).18 Papadimitriou CH (2012) Independence (Patakis Publishers,

Athens).19 Doxiadis A, Papadimitriou CH (2009) Logicomix: An Epic Search

for Truth (Bloomsbury, New York).

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