Self-Consistency of Sequence of Statements: Which is True?

  • Context: MHB 
  • Thread starter Thread starter TheBigBadBen
  • Start date Start date
  • Tags Tags
    Sequence
Click For Summary

Discussion Overview

The discussion revolves around the self-consistency of a sequence of statements that assert the number of false statements among them. Participants explore the implications of these statements for different values of \( n \), examining the conditions under which the statements can be true or false.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a sequence of statements \( S_1 \) to \( S_n \) that claim varying numbers of statements are false, questioning for which \( n \) these statements are self-consistent.
  • Another participant suggests that if \( k \) out of \( n \) statements are true, then the first \( k \) statements must be true and the rest false, implying consistency for any \( n \) and \( 0 \le k \le n \).
  • A participant argues that if \( S_n \) is true, it leads to a contradiction because it would imply \( n \) statements are false, including \( S_n \) itself, thus \( S_n \) must be false.
  • It is noted that if \( n = 1 \), a contradiction arises, while for \( n = 2 \), a consistent solution exists.
  • For \( n \ge 3 \), the reasoning continues that if \( S_{n-1} \) were true, it would imply \( S_{n-1} \) is false, leading to at least two statements being false, thus \( S_2 \) must be true.
  • Participants conclude that a consistent solution exists if and only if \( n \) is even, with the first half of the statements being true and the second half false.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the statements for various values of \( n \). While some agree on the conditions for consistency, others present alternative interpretations, leaving the discussion unresolved regarding the broader implications for odd values of \( n \).

Contextual Notes

The discussion hinges on the logical structure of the statements and their interdependencies, with participants noting contradictions and conditions that affect the truth values without reaching a definitive conclusion on all cases.

TheBigBadBen
Messages
79
Reaction score
0
Consider the following sequence of statements:
$$
S_1: \text{at least 1 of the statements }S_1-S_n \text{ is false}\\
S_2: \text{at least 2 of the statements }S_1-S_n \text{ are false}\\
\vdots \\
S_n: \text{at least } n \text{ of the statements }S_1-S_n \text{ are false}
$$
Where $n$ is some integer.

Question: for which $n$ are these statements self-consistent? In those cases: what is the truth value of each statement?

I got this off of a blog I tend to frequent. I will wait before posting the solution this time.

EDIT:
Changed the question; I had written the statements wrong
 
Last edited:
Physics news on Phys.org
Suppose $k$ out of $n$ statements are true.
Then $S_1$ up to $S_k$ have to be true and the rest has to be false.
This appears to be consistent for any $n$ and any $0\le k \le n$.
 
I like Serena said:
Suppose $k$ out of $n$ statements are true.
Then $S_1$ up to $S_k$ have to be true and the rest has to be false.
This appears to be consistent for any $n$ and any $0\le k \le n$.

Sorry about that, you were absolutely right about the question as phrased.

However, this new version should prove to be a bit more interesting. This is what I had meant; I had accidentally written "true" instead of "false".
 
If $S_n$ is true, then $n$ statements are false, including $S_n$.
Therefore $S_n$ is false.

We now know that at least $1$ statement is false.
Therefore $S_1$ is true.
For $n=1$ this is a contradiction, and for $n=2$ this is a consistent solution.

For $n \ge 3$ we can say, that if $S_{n-1}$ were true, then $n-1$ statements are false.
Since $S_1$ is true, this implies that $S_{n-1}$ is false.
Therefore $S_{n-1}$ is false.

So at least $2$ statements are false.
Therefore $S_2$ is true.
For $n=3$ this is a contradiction, and for $n=4$ this is a consistent solution.

Etcetera.In other words, we get a consistent consistent solution if and only if $n$ is even.
In that case $S_1$ up to $S_{n/2}$ are true and $S_{n/2+1}$ up to $S_{n}$ are false. $\qquad \blacksquare$
 
I like Serena said:
If $S_n$ is true, then $n$ statements are false, including $S_n$.
Therefore $S_n$ is false.

We now know that at least $1$ statement is false.
Therefore $S_1$ is true.
For $n=1$ this is a contradiction, and for $n=2$ this is a consistent solution.

For $n \ge 3$ we can say, that if $S_{n-1}$ were true, then $n-1$ statements are false.
Since $S_1$ is true, this implies that $S_{n-1}$ is false.
Therefore $S_{n-1}$ is false.

So at least $2$ statements are false.
Therefore $S_2$ is true.
For $n=3$ this is a contradiction, and for $n=4$ this is a consistent solution.

Etcetera.In other words, we get a consistent consistent solution if and only if $n$ is even.
In that case $S_1$ up to $S_{n/2}$ are true and $S_{n/2+1}$ up to $S_{n}$ are false. $\qquad \blacksquare$

Couldn't have phrased it better myself.

The source, for anybody interested:
The Parity Paradox – Futility Closet

I highly recommend the website as a time-wasting tool.
 

Similar threads

  • · Replies 20 ·
Replies
20
Views
2K
  • · Replies 27 ·
Replies
27
Views
4K
  • · Replies 18 ·
Replies
18
Views
4K
  • · Replies 3 ·
Replies
3
Views
5K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
2
Views
2K
  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 4 ·
Replies
4
Views
1K