mscroggs.co.uk
mscroggs.co.uk

subscribe

Blog

Logical contradictions

 2016-10-08 
During my Electromagnetic Field talk this year, I spoke about @mathslogicbot, my Twitter bot that is working its way through the tautologies in propositional calculus. My talk included my conjecture that the number of tautologies of length \(n\) is an increasing sequence (except when \(n=8\)). After my talk, Henry Segerman suggested that I also look at the number of contradictions of length \(n\) to look for insights.
A contradiction is the opposite of a tautology: it is a formula that is False for every assignment of truth values to the variables. For example, here are a few contradictions:
$$\neg(a\leftrightarrow a)$$ $$\neg(a\rightarrow a)$$ $$(\neg a\wedge a)$$ $$(\neg a\leftrightarrow a)$$
The first eleven terms of the sequence whose \(n\)th term is the number of contradictions of length \(n\) are:
$$0, 0, 0, 0, 0, 6, 2, 20, 6, 127, 154$$
This sequence is A277275 on OEIS. A list of contractions can be found here.
For the same reasons as the sequence of tautologies, I would expect this sequence to be increasing. Surprisingly, it is not increasing for small values of \(n\), but I again conjecture that it is increasing after a certain point.

Properties of the sequences

There are some properties of the two sequences that we can show. Let \(a(n)\) be the number of tautolgies of length \(n\) and let \(b(n)\) be the number of contradictions of length \(n\).
First, the number of tautologies and contradictions, \(a(n)+b(n)\), (A277276) is an increasing sequence. This is due to the facts that \(a(n+1)\geq b(n)\) and \(b(n+1)\geq a(n)\), as every tautology of length \(n\) becomes a contraction of length \(n+1\) by appending a \(\neg\) to be start and vice versa.
This implies that for each \(n\), at most one of \(a\) and \(b\) can be decreasing at \(n\), as if both were decreasing, then \(a+b\) would be decreasing. Sadly, this doesn't seem to give us a way to prove the conjectures, but it is a small amount of progress towards them.

Similar posts

Logic bot, pt. 2
Logic bot
How OEISbot works
Raspberry Pi weather station

Comments

Comments in green were written by me. Comments in blue were not written by me.
 Add a Comment 


I will only use your email address to reply to your comment (if a reply is needed).

Allowed HTML tags: <br> <a> <small> <b> <i> <s> <sup> <sub> <u> <spoiler> <ul> <ol> <li>
To prove you are not a spam bot, please type "kite" in the box below (case sensitive):

Archive

Show me a random blog post
 2020 

Feb 2020

PhD thesis, chapter ∞
PhD thesis, chapter 5
PhD thesis, chapter 4
PhD thesis, chapter 3
Inverting a matrix
PhD thesis, chapter 2

Jan 2020

PhD thesis, chapter 1
Gaussian elimination
Matrix multiplication
Christmas (2019) is over
 2019 
▼ show ▼
 2018 
▼ show ▼
 2017 
▼ show ▼
 2016 
▼ show ▼
 2015 
▼ show ▼
 2014 
▼ show ▼
 2013 
▼ show ▼
 2012 
▼ show ▼

Tags

golden ratio binary menace matt parker dates puzzles the aperiodical determinants final fantasy reddit manchester science festival wool triangles games map projections sport christmas cross stitch inverse matrices flexagons mathsjam talking maths in public machine learning tennis pythagoras game of life inline code twitter hannah fry bodmas chess logic data visualisation london underground signorini conditions a gamut of games radio 4 cambridge harriss spiral mathslogicbot london big internet math-off hexapawn sorting graph theory go video games advent calendar programming latex news manchester christmas card football platonic solids matrices matrix of cofactors wave scattering world cup golden spiral electromagnetic field javascript royal institution statistics draughts raspberry pi matrix of minors game show probability noughts and crosses geometry arithmetic data approximation gaussian elimination estimation palindromes hats simultaneous equations boundary element methods national lottery php asteroids pac-man preconditioning royal baby dataset braiding propositional calculus countdown ucl bubble bobble stickers phd books numerical analysis curvature oeis weather station sound people maths fractals reuleaux polygons dragon curves martin gardner rhombicuboctahedron chebyshev folding paper rugby gerry anderson folding tube maps bempp realhats error bars frobel mathsteroids nine men's morris computational complexity captain scarlet pizza cutting tmip interpolation sobolev spaces matrix multiplication finite element method speed craft plastic ratio python ternary light polynomials probability coins accuracy european cup misleading statistics trigonometry weak imposition chalkdust magazine

Archive

Show me a random blog post
▼ show ▼
© Matthew Scroggs 2012–2020