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March 24, 2015


How to tell algorithms apart

egdaylightSummerPic

Edgar Daylight was trained both as a computer scientist and as a historian. He writes a historical blog themed for his near-namesake Edsger Dijkstra, titled, “Dijkstra’s Rallying Cry for Generalization.” He is a co-author with Don Knuth of the 2014 book: Algorithmic Barriers Failing: P=NP?, which consists of a series of interviews of Knuth, extending their first book in 2013.

Today I wish to talk about this book, focusing on one aspect.

The book is essentially a conversation between Knuth and Daylight that ranges over Knuth’s many contributions and his many insights.

Knuthbook

One of the most revealing discussions, in my opinion, is Knuth’s discussion of his view of asymptotic analysis. Let’s turn and look at that next.

Asymptotic Analysis

We all know what asymptotic analysis is: Given an algorithm, determine how many operations the algorithm uses in worst case. For example, the naïve matrix product of square {n} by {n} matrices runs in time {O(n^{3})}. Knuth dislikes the use of {O} notation, which he thinks is used often to hide important information.

For example, the correct the count of operations for matrix product is actually

\displaystyle  \frac{1}{3} n^{3} + O(n^{2}).

In general Knuth suggests that we determine, if possible, the number of operations as

\displaystyle  f(n) + O(g(n)),

where {f(n)} and {g(n)} are both explicit functions and {g} is lower-order. The idea is that not only does this indicate more precisely that the number of operations is {\Theta(f(n))}, not just {O(f(n))}, but also is forces us to give the exact constant hiding under the {\Theta}. If the constant is only approached as {n} increases, perhaps the difference can be hidden inside the lower-order {g(n)} term.

An example from the book (page 29) is a discussion of Tony Hoare’s quicksort algorithm. Its running time is {O(n\log n)}, on average. This allows one, as Knuth says, to throw all the details away, including the exact machine model. He goes on to say that he prefers to know:

{\dots} that quicksort makes {2n\ln n - \frac{8}{3}n + O(\log n)} comparisons, on average, and {\frac{1}{3}n \ln n - \frac{4}{9}n + O(\log n)} exchanges, and {\frac{2}{3}n + O(1)} stack adjustments, when sorting random numbers.

The Novelty Game

Theorists create algorithms as one of their favorite activities. A classic way to get a paper accepted into a top conference is to say: In this paper we improve the running time of the best known algorithm for X from order {n^{3}\log n} to {O(n^{3})} by applying methods Y.

But is the algorithm of this paper really new? One possibility is that the analysis of the previous paper was too coarse and the algorithms are actually the same. Or at least equivalent. The above information is logically insufficient to rule out this possibility.

Asymptotic analysis à-la Knuth comes to the rescue. Suppose that we proved that the older algorithm X ran in time

\displaystyle  \frac{1}{4}n^{3}\log n + O(n).

Then we would be able to conclude—without any doubt—that the new algorithm was indeed new. Knuth points this out in the interviews, and adds a comment about practice. Of course losing the logarithmic factor may not yield a better running time in practice, if the hidden constant in {O(n^{3})} is huge. But whatever the constant is, the new algorithm must be new. It must contain some new idea.

This is quite a nice use of analysis of algorithms in my opinion. Knowing that an algorithm contains, for certain, some new idea, may lead to further insights. It may eventually even lead to an algorithm that is better both in theory and in practice.

Open Problems

Daylight’s book is a delight—a pun? As always Knuth has lots to say, and lots of interesting insights. The one caveat about the book is the subtitle: “P=NP?” I wish Knuth had added more comments about this great problem. He does comment on the early history of the problem: for example, explaining how Dick Karp came down to Stanford to talk about his brilliant new paper, and other comments have been preserved in a “Twenty Questions” session from last May. Knuth also reminds us in the book that as reported in the January 1973 issue of SIGACT News, Manny Blum gave odds of 100:1 in a bet with Mike Paterson that P and NP are not equal.

[fixed picture glitch at top]

11 Comments leave one →
  1. Geoffrey Irving permalink
    March 24, 2015 8:23 pm

    There’s a spectrum of precision ranging from coarse theoretical (in P, say) to fine empirical (the algorithm ran in 1831 s). Both ends are useful, but the utility of something in the middle requires more justification. The typical justification for big-O is that it’s the finest measure which is reasonably machine model invariant, and is easy to compute. If you add the constant, the result is typically not invariant, even if you add a bunch of constants: the actual performance of quicksort may depend entirely on the number of cache misses which doesn’t occur in the list.

    However, I wrote the above and then realized that discussing constants in the context of distinguishing between algorithms avoids this complaint entirely. *Any* model which produces a different constant between two algorithms is strong evidence that the algorithms are different, which is a lovely way to think about constants and their use. Thanks!

    • Serge permalink
      March 31, 2015 6:19 pm

      Quite right. Complexity classes are nothing but a scientific-looking notation for the current state of our knowledge about algorithms. Just because a class of problems is independent of a computing model, that doesn’t make it more real.

  2. March 24, 2015 9:30 pm

    Dick Lipton reminds us that “as reported in the January 1973 issue of SIGACT News, Manny Blum gave odds of 100:1 in a bet with Mike Paterson that P and NP are not equal.”

    Thi iis one of those wagers for which we can enjoyably (to borrow the title of one of Scott Aaronson’s columns) “read the Fine Print”.

    E.g., it would be logically consistent to wager that the following propositions are either both true or both false:

    Prop A  P and NP are not equal (e.g., in ZFC).

    Prop B  P and NP are not provably unequal (e.g, in HoTT).

    We reason further, that the discordant world !(Prob A) & (Prob B) is scarcely credible, on the grounds that long-established mathematical edifices like the polynomial hierarchy and the complexity zoo alike would vanish. Similarly, to arrive at the discordant world of (Prob A) & !(Prob B), a considerable set of known complexity-theoretic obstructions would have to be surmounted.

    Conclusion  It’s a reasonably safe bet that Prob A and Prob B have the same truth-value! Which is easier to prove/disprove? Don’t ask me!

    Natural Question  What were the precise terms of the Blum-Paterson wager?

    The world wonders. Students and wager-holders especially!

  3. MadHatter permalink
    March 25, 2015 6:04 am

    Not sure I understand. Suppose the upper bound on the running time of the older algorithm for X was 1/4 * n^3 * log(n) + O(n). It could be that the true running time of the *same* algorithm is O(n^3), which may be is what is proved in the newer paper. So, it’s not necessary that the new paper give a different algorithm…

  4. March 25, 2015 2:16 pm

    At the end of the famous paper of Steven Cook about NP-completeness there is an interesting discussion about the measurement of complexity. Cook suggests that we could probably add more variables at the mathematical expressions that discribe complexity and that a new definition of complexity could open new roads to reshearch. Looking at your post and Cook’s paper makes me think that more reshearch about the measurement of complexity should be done.

  5. March 25, 2015 10:10 pm

    There’s also a short video in which Donald Knuth elaborates a little bit on his answer of question 17 from the “Twenty Questions” session.

    • March 25, 2015 11:06 pm

      Thanks. It is mainly the “Loss of Innocence of P” answer—a phrase I heard ascribed by Person Y to Person X in the mid/late-1980s where I think X U Y = {Paul Seymour, Neil Robertson, Rod Downey, Mike Fellows, Dominic Welsh}. But I think this possibility has been digested by many people still strongly believing P != NP.

  6. E.L. Wisty permalink
    March 27, 2015 6:05 am

    Reblogged this on Pink Iguana.

  7. Chidambaram Annamalai permalink
    April 5, 2015 4:32 pm

    The reason computer scientists use the coarser estimate is that in most cases they have reason to believe that the answer (whether it be the running time analysis of an algorithm, upper /lower bound on some quantity) is not the “right” one and they expect improvements to follow. In such cases rushing to find the lower order terms or even the hidden constants is misguided and a classic case of premature optimization.

    Once there is consensus that the right answer has been reached– a la quicksort– then this makes sense.

  8. Eulog-i permalink
    November 22, 2015 9:46 am

    P = NP (PROVEN)

    P = NP, in the factorization of all odd number.

    The P=NP problem has been demonstrated by Andri Lopez.

    It fixes a problem that existed in the computer, that stop when (N) is a prime number. It does by sterting the algorithm with a set of equations that make the computer will stop, if (N) is prime and if it is not, continue to define the two factors (N).

    Published in: http://www.ijma.info Vol 6 (9) 2015

  9. Jhon Hard permalink
    November 22, 2015 9:57 am

    P=NP problem (proven)

    P =NP, in the factorization of all odd number.

    The P=NP problem has been demonstrated by Andri Lopez.

    It fixes a problem that existed in the computer, that stop when (N) is a prime number. It does by starting the algorithm with a set of equations that make the computer will stop, if (N) is prime and if it is not, continue to define the two factors (N).

    Published in: http://www.ijma.info Vol 6 (9) 2015

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