uma discussao interessante no blog do Litpon.

---------- Forwarded message ---------
From: Gödel's Lost Letter and P=NP <[email protected]>
Date: Sun, Aug 23, 2020 at 11:12 PM
Subject: Weekly digest for Gödel's Lost Letter and P=NP, on August 24, 2020
To: <[email protected]>


rjlipton posted: " If you cannot find proofs, talk about them. Robert
Reckhow with his advsior Stephen Cook famously started the formal study of
the complexity of proofs with their 1979 paper. They were interested in the
length of the"

New post on *Gödel's Lost Letter and P=NP*
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Logical
Complexity of Proofs
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by
rjlipton
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*If you cannot find proofs, talk about them.*
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Robert Reckhow with his advsior Stephen Cook famously started the formal
study of the complexity of proofs with their 1979 paper
<https://www.cs.toronto.edu/~sacook/homepage/cook_reckhow.pdf>. They were
interested in the length of the shortest proofs
<https://en.wikipedia.org/wiki/Proof_complexity> of propositional
statements. Georg Kreisel and others may have looked at proof length
earlier, but one of the key insights of Reckhow and Cook is that low level
propositional logic is important.

Today I thought we might look at the complexity of proofs.

Cook and Reckhow were motivated by issues like: How hard is it to prove
that a graph has no clique of a certain size? Or how hard to prove that
some program halts on all inputs of length [image: {n}]? All of these
questions ask about the length of proofs in a precise sense. Proofs have
been around forever, back to Euclid at least, but Cook and Reckhow were the
first to formally study the lengths of proofs.

They were not directly interested in actual proofs. The kind you can find
in the arXiv <https://arxiv.org/archive/math> or in a math journal, or at a
conference---online or not. The kind that are in their paper.
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We are talking today about these types of proofs. Not proofs that graphs
have cliques. But proofs that a no planar graph can have a [image: {5}]
clique.

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Proofs

Proofs are what we strive to find ever day. They the coin that measures
progress in a mathematical field like complexity theory. We do sometimes
work out examples, sometimes do computations to confirm conjectures on
small examples, sometimes consider analogies to other proofs. But mostly we
want to understand proofs. We want to create new ones and understand others
proofs.

Years ago when studying the graph isomorphism problem, I did some extensive
computations for the random case. That is for the case of isomorphism for a
random dense graphs against a worst case other graph. The computations
helped me improve my result. It did not yield a proof, of course, but
helped me realize that a certain lemma could be improved from a bound [image:
{\log n}] to [image: {O(1)}]. My results were dominated by paper
<https://www.researchgate.net/profile/Stanley_Selkow/publication/220618511_Random_Graph_Isomorphism/links/00463537d337e6a35d000000/Random-Graph-Isomorphism.pdf>
of Laszlo Babai, Paul Erdös, and Stanley Selkow. Oh well.

Proofs Complexity

There are several measures of complexity for proofs. One is the length.
Long proofs are difficult to find, difficult to write up, difficult to
read, and difficult to check. Another less obvious measure is the logical
structure of a proof. What does this mean?

Our idea is that a proof can be modeled by a formula from propositional
logic. The [image: {P}] is what we are trying to prove and the letters [image:
{A}] and so on are for statements we already know.

   - [image: {(A \rightarrow P)}] This is a direct proof.
   - [image: {(\neg P \rightarrow \neg A)}] This is a proof by
   contradiction.
   - [image: {( A \vee \neg A \rightarrow P)}] This is proof that uses a
   statement [image: {A}] that may be true or false.

   The last is a slight cheat, we use [image: {A \vee \neg A}] to stand for
   a kind of axiom. A perfect example is from number theory. Let [image:
   {\pi(X)}] be the number of primes less than [image: {x}] and the
   function [image: {li(x)}] the logarithmic function
   
<https://en.wikipedia.org/w/index.php?title=Logarithmic_integral_function&action=edit&section=1>.


   [image: \displaystyle li(x) = \int_0^x \frac{dt}{\ln t}.]

   The prime number theorem
   <https://en.wikipedia.org/wiki/Prime_number_theorem> says that

   [image: \displaystyle \pi(x) = li(x) + E(x),]

   an error term.


   
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   It was noted that [image: {li(x)}] is larger than [image: {\pi(x)}] for
   known values. The obvious question was that could

   [image: \displaystyle li(x) \ge \pi(x),]

   be always true? If so this would be an interesting inequality. In 1914
   John Littlewood famously proved
   
<https://www.google.com/books/edition/_/2SUrpE8NK6sC?hl=en&gbpv=1&pg=PA33&dq=John+Littlewood+pi+nd+li>
   that this was not true:

   *Theorem 1* * If the Riemann Hypothesis is true: *

   [image: \displaystyle \pi(x) - li(x)]

   is infinitely often positive and negative. If the Riemann Hypothesis is
   false:

   [image: \displaystyle \pi(x) - li(x)]

   is infinitely often positive and negative.

   Thus he proved that

   [image: \displaystyle \pi(x) - li(x)]

   is infinitely often positive and negative whether the the Riemann is
   true or not.

   Proofs in Trouble

   A sign of a proof in danger is, in my opinion, is not just the length. A
   better measure I think is the logical flow of proof. I know of no actual
   proof that uses this structure:

   [image: \displaystyle (A \rightarrow B) \rightarrow ((A \vee C)
   \rightarrow (B \vee C))]

   Do you? Even if your proof is only a few lines or even pages, if the
   high level flow was the above tautology I would be worried.

   Another example is [image: {P \rightarrow P}]. This of course is a
   circular proof. It seems hard to believe we would actually do this, but it
   has happen. The key is that no one says: I will assume the theorem to prove
   it. The flaw is disguised better than that.

   I cannot formally define this measure. Perhaps it is known, but I do
   think that it would be an additional measure. For actual proofs, ones we
   use every day, perhaps it would be valuable. I know I have looked at an
   attempted proof of X and noticed the logical flow in this sense was too
   complex. So complex that it was wrong. The author of the potential proof
   was me.

   Open Problems

   Is this measure, the logical flow of a proof, of any interest?


*rjlipton
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| August 19, 2020 at 8:00 am | Tags: complexity proof
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Logic
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logical flow
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proof length
<https://public-api.wordpress.com/bar/?stat=groovemails-events&bin=wpcom_email_click&redirect_to=https%3A%2F%2Frjlipton.wordpress.com%2Ftag%2Fproof-length%2F&sr=1&signature=f51e6591deb39223cfc71932ba0e4f06&user=4123301&_e=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&_z=z>
| Categories: Oldies
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Proofs
<https://public-api.wordpress.com/bar/?stat=groovemails-events&bin=wpcom_email_click&redirect_to=https%3A%2F%2Frjlipton.wordpress.com%2Fcategory%2Fall-posts%2Fproofs%2F&sr=1&signature=535e031ecd1dbdee0d446b137a06473e&user=4123301&_e=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&_z=z>,
Results
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Valeria de Paiva
http://vcvpaiva.github.io/
http://www.cs.bham.ac.uk/~vdp/

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