|Mathematica Tutorial||Functions »|Tutorials »|More About »|
|NHoldAll||prevent any arguments of a function from being affected by N|
|NHoldFirst||prevent the first argument from being affected|
|NHoldRest||prevent all but the first argument from being affected|
Usually N goes inside functions and gets applied to each of their arguments.
This tells Mathematica not to apply N to the first argument of f.