  We discuss the appearance of odd-frequency spin-triplet s-wave
superconductivity, first proposed by Berezinskii [{\it JETP} {\bf 20}, 287
(1974)], on the surface of a topological insulator proximity coupled to a
conventional spin-singlet s-wave superconductor. Using both analytical and
numerical methods we show that this disorder robust odd-frequency state is
present whenever there is an in-surface gradient in the proximity induced gap,
including superconductor-normal state (SN) junctions. The time-independent
order parameter for the odd-frequency superconductor is proportional to the
in-surface gap gradient. The induced odd-frequency component does not produce
any low-energy states.
