proof of Ruffa’s formula for continuous functions
Define to be the following sum:
Making the substitution and using the fact that when is odd to express the sum over even values of as a sum over all values of , this becomes
Subtracting this sum from and simplifying gives
Using the telescoping sum trick, we may write
To complete the proof, we must investigate the limit as . Since is assumed continuous
and the interval is compact
, is uniformly continuous
. This means that, for every , there exists a such that implies . By the Archimedean property, there exists an integer such that . Hence, when lies in the interval . Thus, is a Darboux upper sum for the integral
and is a Darboux lower sum. (Darboux’s definition of the integral may be thought of as a modern incarnation of the ancient method of exhaustion.) Hence
Taking the limit , we see that