Jupyter notebook trick: recall last output cells

[Jay Wang]

In Jupyter notebook, it's often handy to use the last few output cells without variable name reference. Here is how:
_: (underscore) recall the last output
__: (double underscore) recall the second last output
_x: recall the output number x

Example
In  [1]: a = 0.5
         a
Out [1]: 0.5
In  [2]: _
Out [2]: 0.5
In  [3]: __
Out [3]: 0.5
In  [4]: _1
Out [4]: 0.5
In  [5]: 3*_1
Out [5]: 1.5