Past exam of the mathematics course of the University of Oxford / 2015 / Prelims / Mathematics II / 1 / b / i / Solution

Choose the sequenceEvery even index is a peak of a sequence $(a_n)$, since its value is $1$ and all later values are at most $1$. Every odd index is a peak of a sequence $(-a_n)$ for the same reason. Both sets of indices are infinite.