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For questions requesting examples of a certain structure or phenomenon

2 votes

Examples of conjectures whose direct falsity implies different consequences than indirect fa...

Here's an example from computability theory. Statement $A$:"The Turing degrees are linearly ordered". Statement $B$: "The $\Sigma^0_1$ Turing degrees are linearly ordered". Statement $A$ was refut …
Bjørn Kjos-Hanssen's user avatar
2 votes

Examples of $G_\delta$ sets

The set $\mathbb R\setminus\mathbb Q$ of all irrational numbers.
Bjørn Kjos-Hanssen's user avatar
58 votes

Is there a measure zero set which isn't meagre?

Let $p_i$ be a list of the rational numbers. Let $U_{i,n}$ be an open interval centered on $p_i$ of length $2^{-i}/n$. Then $V_n=\cup_i U_{i,n}$ is an open cover of the rationals, of measure at most $ …
Bjørn Kjos-Hanssen's user avatar
13 votes
Accepted

Example of a function that behaves like another function

This may be more than 15 symbols, but $$ f(x)= x + \frac{1}{1+(x+\delta-1)^{2n/(2n-1)}} $$ where $\delta>0$ is small and $n$ is large. For $n=2$ and $\delta=1/10$ it looks like this: (source) ED …
Bjørn Kjos-Hanssen's user avatar