Here is a sequence that is quite famous but not in OEIS I think (perhaps because too few elements are known). A hypergraph is $v$-uniform if every edge has $v$ vertices, and 2-colourable if the vertices can be coloured using two colours so that no edge is monochromatic (this is also called "Property B").

Now define $m(v)$ to be the least number of edges in a $v$-uniform hypergraph that is not 2-colourable.

Erdős and Hajnal noted about a million years ago that $m(1)=1$, $m(2)=3$ and $m(3)=7$. In 2014 Patric R.J. Östergård proved $m(4)=23$ using an extensive computation. No other values are known.

Here is a sequence that is quite famous but not in OEIS I think (perhaps because too few elements are known). A hypergraph is $v$-uniform if every edge has $v$ vertices, and 2-colourable if the vertices can be coloured using two colours so that no edge is monochromatic (this is also called "Property B").

Now define $m(v)$ to be the least number of edges in a $v$-uniform hypergraph that is not 2-colourable.

Erdős and Hajnal noted about a million years ago that $m(1)=1$, $m(2)=3$ and $m(3)=7$. In 2014 Patric R.J. Östergård proved $m(4)=23$ using an extensive computation. No other values are known.