In TeX for example we type x_n for subscript $x_n$. Unfortunately you cannot use the underscore for that purpose in Mathematica, since it has a totally different meaning. Compare
TeXForm[Subscript[x, n]] MathMLForm[Subscript[x, n]]
it would only apply to computer language...what am looking is the mathematical expression,say when doing a number raised to a power, we simply keyin..n^3....thanks for responding...
Have you already seen the documentation for Subscript?
