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Israel is holding an election Tuesday. It’s a multi-party system in Israel, based on proportional representation, and the main battle is between two blocs of parties. We call them the left-wing bloc and the right-wing bloc, and civics textbooks will tell you that this division is about policy: the Arab-Israeli conflict, economics and the role of religion. But let’s face it, nobody cares about public policy anymore. It’s all about Bibi. Parties that intend to form a coalition under Bibi belong to the right-wing bloc and parties that intend to rid the country of him belong to the left wing bloc.

In addition to the battle between the blocs, there is also a contest between the parties in each bloc, especially between one big party and several satellites. Bibi, in particular, is expected to use the final days of the campaign to siphon votes from satellites to his own Likud party, as he did in previous elections. Indeed, in recent days, Bibi and his social media fans said that the number of seats of the Likud party compared to the major opponent (called BlueWhite), and not the size of the blocs, will determine whether Bibi will form the government again. BlueWhite makes the same argument to left-wing voters.

But if Bibi is serious about cannibalizing his bloc, he could use a more fatal argument by appealing to the electoral threshold, that is the minimum share of total votes that a party has to achieve to be entitled to seats in the parliament. The current threshold 3.25%, which translates to roughly five seats in the 120-seat parliament. This sounds like a low threshold, but there are eleven parties in the current parliament, some of them have split into two, there are new parties that are serious contenders, and Likud and BlueWhite together are expected to win about half of the total votes. So many small parties are close to the threshold in the opinion polls. Since the two blocs are almost tied (with a small advantage to the right-wing bloc), the electoral threshold might end up playing a significant role in determining the outcome if many votes are cast to a party that doesn’t cross the threshold.

Enter game theory

It is difficult to do a game-theoretic analysis of anything election because we don’t have a good explanation for what drives voters to stand in line in the voting booth rather than go to the beach. Let me bypass this issue and just assume that you gain some utility from voting to your favorite party. So assume for simplicity that right-wing voters have two options, Likud or New Right (NR), which is one of the new parties in the right-wing bloc, whose leaders Bibi detests. According to the polls, 4% of the voters are New Right supporters. These guys will get utility 2 if they vote New Right and utility 1 if they vote Likud. And here is the twist: you only get this utility if the party you vote to passes the electoral threshold. If it doesn’t, you feel like your vote has been wasted and you get utility 0.

Whether or not an NR supporter will vote her preference depends on what she thinks the other supporter will do. In fact, NR supporters are involved in a population stag hunt game (sometimes called investment game). There are two Nash equilibria in the game: either everyone votes NR or everyone votes Likud. The first equilibrium requires players to take the risky action of voting NR, and if they all go along, they all get high utility 2. I have played variants of the investment game in class several times. Typically, players go for the safe choice, which in this case is Likud. If you do this, you guarantee yourself a utility 1 regardless of what the other players do.

How could the potential voters of NR coordinate on the risky equilibrium which gives a higher utility? I think the public opinion polls have a big role here. In the recent polls, NR has been consistently hovering above the threshold. The polls make it commonly known that there are sufficiently many potential NR voters, and also that they plan to go along with voting NR.

Suppose however that on Monday Bibi hints that his internal polls show that NR does not cross the threshold. What would our NR supporter do? She will likely know that there are no such polls since most Bibi supporters know that he is a lier. But perhaps other players will believe Bibi and switch to voting Likud, which will drive NR below the threshold. Or perhaps other players will worry that some other players will believe him. You see where this is going: to play the risky equilibrium, you need confidence that the other players are playing it too. Bibi can shake this confidence even if he can’t change the fact that NR has enough supporters to cross the threshold if they all collaborate.

By the way, I believe that established parties that are already represented in the parliament are less vulnerable to such an attack, since the confidence of their supporters in the risky equilibrium is stronger, as they played it once already.

Go ahead Bibi, make my day.

Apparently, it is quite the rage to festoon one’s slides with company logos, particularly of the  frightful five. At present this is done for free. It suggests a new business. A platform that matches advertisers to faculty. Faculty can offer up their slides and advertisers can bid for the right to place their logos on the slides.

After presenting Richard Weber’s remarkable proof of Gittins’ index theorem in my dynamic optimization class, I claimed that the best way to make sure that you understand a proof is to identify where the assumptions of the theorem are used. Here is the proof again, slightly modified from Weber’s paper, followed by the question I gave in class.

First, an arm or a bandit process is given by a countable state space S, a transition function q(s'|s) and a payoff function r:S\rightarrow [0,1]. The interpretation is that at every period, when the arm is at state s, playing it gives a reward r(s) and the arm’s state changes according to q(\cdot|s).

In the multi-armed bandit problem, at every period you choose an arm to play. The states of the arms you didn’t choose remain fixed. Your goal is to maximize expected total discounted rewards. Gittins’ theorem says that for each arm there exists a function \gamma:S\rightarrow [0,1] called the Gittins Index (GI from now on) such that, in a multi armed problem, the optimal strategy is to play at each period the arm whose current state has the largest GI. In fancy words, the theorem establishes that the choice which arm to play at each period satisfies Independent of Irrelevance Alternatives: Suppose there are three arms A,B,C whose current states are a,b,c. If you were going to start by playing A if only A and B were available, then you should not start with B when A,B,C are available.

The proof proceeds in several steps:

  1. Define the Gittins Index at state s to be the amount \gamma such that, if the casino charges \gamma every time you play the arm, then both playing and not playing are optimal actions at the state s. We need to prove that there exists a unique such \gamma. This is not completely obvious, but can be shown by appealing to standard dynamic programming arguments.
  2. Assume that you enter a casino with a single arm at some state s with GI \gamma. Assume also that the casino charges \gamma every time you play the arm. At every period, you can play, or quit playing, or take a break. From step 1, it follows that regardless of your strategy, the casino will always get a nonnegative net expected net payoff, and if you play optimally then the net expected payoff to the casino (and therefore also to you) is zero. For this reason this \gamma (the GI of the initial state) is called the fair charge. Here, playing optimally means that you either not play at all or start playing and continue to play every period until the arm reaches a state with GI strictly smaller then \gamma, in which case you must quit. It is important that as long as the arm is at a state with GI strictly greater than \gamma you continue playing. If you need to take a restroom break you must wait until the arm reaches a state with GI \le \gamma.
  3. Continuing with a single arm, assume now that the casino announces a new policy that at every period, if the arm reaches a state with GI that is strictly smaller than the GI of all previous states, then the charge for playing the arm drops to the new GI. We call these new (random) charges the prevailing charges. Again, the casino will always get a nonnegative net expected payoff, and if you play optimally then the net expected payoff is zero. Here, playing optimally means that you either not play at all or start playing and continue to play foreover. You can quit or take a bathroom break only at periods in which the prevailing charge equals the GI of the current state.
  4. Consider now the multi-arms problem, and assume again that in order to play an arm you have to pay its current prevailing charge as defined in step 3. Then again, regardless of how you play, the Casino will get a nonnegative net payoff (since by step 3 this is the case for every arm separately), and you can still get an expected net payoff 0 if you play optimally. Playing optimally means that you either not play or start playing. If you start playing you can quit, take a break, or switch to another arm only in periods in which the prevailing charge of the arm you are currently playing equals the GI of its current state.
  5. Forget for a moment about your profits and assume that what you care about is maximizing payments to the casino (I don’t mean net payoff, I mean just the charges that the casino receives from your playing). Since the sequence of prevailing charges of every arm is decreasing, and since the discount factor makes the casino like higher payments early, the Gittins strategy — the one in which you play at each period the arm with highest current GI, which by definition of the prevailing charge is also the arm with highest current prevailing charge — is the one that maximizes the Casino’s payments. In fact, this would be the case even if you knew the realization of the charges sequence in advance.
  6. The Gittins strategy is one of the optimal strategies from step 4. Therefore, its net expected payoff is 0.
  7. Therefore, for every strategy \sigma,
    Rewards from \sigma<= Charges from \sigma<= Charges from Gittins strategy
    (First inequality is step 4 and second is step 5)
    And Charges from Gittins strategy = Rewards from Gittins Strategy
    (step 6)
  8. Therefore, Gittins strategy gives the optimal possible total rewards.

That’s it.

Now, here is the question. Suppose that instead of arms we would have dynamic optimization problems, each given by a state space, an action space, a transition function, and a payoff function. Let’s call them projects. The difference between a project and an arm is that when you decide to work on a project you also decide which action to take, and the current reward and next state depend on the current state and on your action. Now read again the proof with projects in mind. Every time I said “play arm i”, what I meant is work on project i and choose the optimal action. We can still define an “index”, as in the first step: the unique charge \gamma such that, if you need to pay \gamma every period you work on the project (using one of the actions) then both not working and working with some action are optimal. The conclusion is not true for the projects problem though. At which step does the argument break down ?

Volume 42 of the AER, published in 1952, contains an article by Paul Samuelson entitled `Spatial Price Equilibrium and Linear Programming’. In it, Samuelson uses a model of Enke (1951) as a vehicle to introduce the usefulness of linear programming techniques to Economists. The second paragraph of the paper is as follows:

In recent years economists have begun to hear about a new type of theory called linear programming. Developed by such mathematicians as G. B. Dantzig, J. v. Neumann, A. W. Tucker, and G. W. Brown, and by such economists as R. Dorfman, T. C. Koopmans, W. Leontief, and others, this field admirably illustrates the failure of marginal equalization as a rule for defining equilibrium. A number of books and articles on this subject are beginning to appear. It is the modest purpose of the following discussion to present a classical economics problem which illustrates many of the characteristics of linear programming. However, the problem is of economic interest for its own sake and because of its ancient heritage.

Of interest are the 5 reasons that Samuelson gives for why readers of the AER should care.

  1. This viewpoint might aid in the choice of convergent numerical iterations to a solution.

  2. From the extensive theory of maxima, it enables us immediately to evaluate the sign of various comparative-statics changes. (E.g., an increase in net supply at any point can never in a stable system decrease the region’s exports.)

  3. By establishing an equivalence between the Enke problem and a maximum problem, we may be able to use the known electric devices for solving the former to solve still other maximum problems, and perhaps some of the linear programming type.

  4. The maximum problem under consideration is of interest because of its unusual type: it involves in an essential way such non-analytic functions as absolute value of X, which has a discontinuous derivative and a corner; this makes it different from the conventionally studied types and somewhat similar to the inequality problems met with in linear programming.

  5. Finally, there is general methodological and mathematical interest in the question of the conditions under which a given equilibrium problem can be significantly related to a maximum or minimum problem.


My students often use me as a sounding board for their new ventures. A sign that the modern University could pass for a hedge fund with classrooms. The request brings a chuckle as it always reminds me of my first exposure to entrepreneurial activity.

It happened in the most unlikeliest of places as well as times. A public (i.e. private) school in pre-Thatcherite England.  England was then the sick man of Europe and its decline was blamed upon the public schools.  Martin Wiener’s English Culture and the Decline of the Industrial Spirit, for example, argued that the schools had turned a nation of shopkeepers into one of lotus eaters.

Among the boys was a fellow, I’ll call Hodge. He was a well established source of contraband like cigarettes and pornographic magazines. He operated out of a warren of toilets in the middle of the school grounds called the White City. Why the school needed a small building devoted entirely to toilets was a product of the English distrust of indoor plumbing and central heating.

One lesson I learnt from Hodge was never buy a pornographic magazine sight unseen. The Romans call it caveat emptor, but, I think this, more vivid.

Hodge was always on the look out for new goods and services that he could offer for a profit to the other boys. One day, he hit upon the idea of buying a rubber woman (it was plastic and inflatable) and renting it out.  The customer base consisted of 400 teenage boys confined to a penal colony upon a wind blasted heath.

Consider the challenges. How was he to procure one (no internet)? Where would he hide the plastic inamorata to prevent theft or confiscation by the authorities? How would he find customers (no smart phones)? What should he charge? What was to prevent competition? And, of course, what happened? All, I think, best left to the imagination.



There is a test for `smarts’ that Sir Peter Medawar was fond of. I often think of it when teaching equilibrium.

If you have ever seen an El Greco, you will notice that the figures and faces are excessively elongated. Here is an example.El_Greco_-_Portrait_of_a_Man_-_WGA10554The eye surgeon Patrick Trevor-Roper, brother to the historian Hugh offered an explanation. Readers of  certain vintage will recall the long running feud between Hugh Trevor-Roper and Evelyn Waugh. Waugh said that the best thing Hugh Trevor-Roper could do would be to change his name and leave Oxford for Cambridge. Hugh Trevor-Roper eventually  became Lord Dacre and left Oxford for Cambridge. But, I digress.

Returning to Patrick, he suggested that El Greco had a form of astigmatism, which distorted his vision and led to elongated images forming on his retina. Medawar’s question was simple: was Patrick Trevor-Roper correct?

In a CS paper, it is common to refer to prior work like [1] and [42] rather than Brown & Bunter (1923) or Nonesuch (2001). It is a convention I have followed in my papers with CS colleagues. Upon reflection, I find it irritating and mean spirited.

  1. No useful information is conveyed by the string of numbers masquerading as references beyond the statement: `authors think there are X relevant references.’
  2. A referee wishing to check if the authors are aware of relevant work must scroll or leaf to the end of the paper to verify this.
  3. The casual reader cannot be surprised by some new and relevant reference unless they scroll or leaf to the end of the paper to verify this.
  4. Citations are part of the currency (or drug) we live by. Why be parsimonious in acknowledging the contributions of A. N. Other? It shows a want of fellow feeling.

I suspect that the convention is an artifact of the page limits on conference proceedings. A constraint that seems quaint. Some journals, the JCSS for example, follows the odd convention of referring to earlier work as Bede [22]! But which paper by the venerable and prolific Bede does the author have in mind?


Many people say (actually, just one) that the Republican’s have a plan to remove Trump from the Presidency, should he win in November using the 25th amendment. Section 4 of the amendment reads:

`Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.’

The VP is Pence. The President pro tempore of the Senate, is the senior senator of the majority party and Paul Ryan is the Speaker of the House.
The President can object. At which point, Congress resolves the matter, specifically,

`….two-thirds vote of both Houses that the President is unable to discharge the powers and duties of his office, the Vice President shall continue to discharge the same as Acting President; otherwise, the President shall resume the powers and duties of his office.’


It is not often that Terry Tao gets into politics in his blog, but, as political observers like to say, normal rules don’t apply this year. Tao writes that many of Trump’s supporters secretly believe that he is not even remotedly qualified for the presidency, but they continue to entertain this possibility because their fellow citizens and the media and politicians seem to be doing so. He suggests that more people should come out and reveal their secret beliefs.

I generally agree with Tao’ sentiment and argument, but I have a quibble. Tao describe the current situation as mutual knowledge without common knowledge. This, I think, is wrong. To get politics out of the way, let me explain my position using a similar situation which Tao also mentions: The Emperor’s new clothes. I have already come across people casting the Emperor’s story in terms of mutual knowledge without common knowledge, and I think it is also wrong. The way I understand the story, before the kid shouts, each of the Emperor’s subjects sees that the Emperor is naked, but after observing everybody else’s reaction, each subject updates her own initial belief and deduces that she was probably wrong. The subjects now don’t think that the Emperor is naked. Rather, each subjects thinks that her own eyes deceived her.

But when game theorists and logicians say that an assertion is mutual knowledge (or mutual belief) we mean that each of us, after taking into account our own information including what we deduce about other people’s information, think the assertion is true. In my reading of the Emperor’s new cloths story this is not the case.

For an assertion to be common knowledge, we need in addition that everybody knows that everybody knows that the assertion is true, and that everybody knows that everybody knows that everybody knows that the assertion is true, and onwards to infinity. A good example of a situation with mutual knowledge and no common knowledge is the blue-eyed islanders puzzle (using the story as it appears Terrence’ blog and a big spoiler ahead if you are not familiar with the puzzle): Before the foreigner makes an announcement, it is mutual knowledge that there are at least 99 blue-eyed islanders, but this fact is not common knowledge: If Alice and Bob are both blue-eyed then Alice, not knowing the color of her own eyes, thinks that Bob might observe only 98 blue-eyed islanders. In fact it is not even common knowledge that there are at least 98 blue-eyed Islanders, because Alice thinks that Bob might think that Craig might only observe 97 blue-eyed Islanders. By similar reasoning, before the foreigner’s announcement, it is not even common knowledge that there is at least one blue-eyed islander. Once the foreigner announces it, this fact becomes common knowledge.

No mutual knowledge and no common knowledge are two situations that can have different behavioral implications. Suppose that we offer each of the subjects the following private voting game: Is the emperor wearing clothes ? You have to answer yes or no. If you answer correctly you get a free ice cream sandwich, otherwise you get nothing. According to my reading of the story they will all give the wrong answer, and get nothing. On the other hand, suppose you offer a similar game to the islanders — even before the foreigner arrives — Do you think that there is at least one blue-eyed islander ?  they will answer correctly.

There is an alternative reading of the Emperor’s story, according to which it is indeed a story about mutual knowledge without common knowledge: Even after observing the crowd’s reaction, each subject still knows that the Emperor is naked, but she keeps her mouth shut because she suspects that her fellow subjects don’t realize it and she doesn’t want to make a fool of herself. This reading strikes me as less psychologically interesting, but, more importantly, if that’s how you understand the story then there is nothing to worry about. All the subjects will vote correctly anyway and get the ice cream even without the little kid making it a common knowledge. And Trump will not be elected president even if people continue to keep their mouth shut.

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