You may notice that the function Part[expr, n]
used to pick out the nth
term in a sum is the same as the function described in "Manipulating Elements of Lists"
for picking out elements in lists. This is no coincidence. In fact, as discussed in "Manipulating Expressions like Lists,"
expression can be manipulated structurally much like a list. However, as discussed in "Manipulating Expressions like Lists,"
you must be careful, because Mathematica
often shows algebraic expressions in a form that is different from the way it treats them internally.