  The uncountability of the real numbers is one of their most basic properties,
known (far) outside of mathematics. Cantor's 1874 proof of the uncountability
of the real numbers even appears in the very first paper on set theory, i.e. a
historical milestone. Despite this famous status and history, the computational
properties of the uncountability of the real numbers have not been studied
much. In this paper, we study the following computational operation that
witnesses that the real numbers not countable:
  on input a countable set of reals, output a real not in that set.
  In particular, we formulate a considerable number of operations that are
computationally equivalent to the centred operation, working in Kleene's
higher-order computability theory based on his S1-S9 computation schemes.
Perhaps surprisingly, our equivalent operations involve most basic properties
of the Riemann integral and Volterra's early work circa 1881.
