Random Clocks on Computation Paths]{Random Clocks on Computation Paths: How Far Can They Reach, and When Does Stepping Back Help?

Overview & Synopsis

We watch a computation only at moments chosen by a random clock that can leap ahead, pause, or step back, and we count looks rather than steps. How far can such a clock reliably see with polynomially many looks, and does stepping back help? Rare long leaps reach exponential and doubly exponential time, with infinitely many levels between polynomial and exponential time. A clock that tends to fall back sees only the start of the input, and below polynomial time we find infinitely many incomparable levels. Stepping back can help decisively. For clocks whose long leaps occur only at rare positions, no forward-only clock goes beyond polynomial time, while even single steps back raise the reach to exponential time. Stepping back cannot help, however, when the clock's leaps ignore its position.