A finite series is one with a finite number of terms and, for example, when
using the summation notation, the finite series a3/a4/. . ./a20 would be written
The summation index n is a dummy index, in the sense that it may be
replaced by any other symbol without changing the meaning of the summa-
tion, and thus
an X r1
ar . . . X s1
The following are four examples of infinite series with numerical terms.