Create a virtual table with numbers

This article describes how you can create a virtual table with increasing number.
So a table with contents:

1
2
3
4
5
...
There are many ways to do this. For example:
* A real table with increasing number.

* select rownum on all_objects where rownum < 1000;

* A pipelined function that returns numbers from a for loop

* select rownum
from (SELECT 1 FROM dual GROUP BY CUBE(1,2,3,4,5,6) )
where rownum < 50;
* SELECT level
FROM dual
CONNECT BY LEVEL