StoneTemplePython said:
Your 'counterexample' doesn't hold water I'm afraid. If you place two canister touching each other (I guess that's what 'together' means?) then it's just another legal 4 canister configuration. If together means 'exactly the same spot' I'm not really sure that's allowed physically but in the interest of sport I'd point out that if it were allowed you have just reduced it to the 2 canister problem which most people can solve by inspection.
"At the same spot
relative to the track" is what I meant. The canisters could be stacked on top of each other, for example.
As for my counter-example, I mixed my problem statements and was still thinking in terms of the one I working on. I interpreted "extra fuel" to mean more than 1/Nth of the total fuel, rather than more than 100% of the fuel needed for the next section. It took me a while to even see this.
Reading your problem description more carefully, I see you asked if there was a viable starting point, not to identify which point that might be. The reduction proof works for identifying that a starting point exists, but not for identifying which it is. Oddly enough, it is not the one with the most fuel or even the most percentage of fuel for its leg. Consider N=4 where all canisters are spaced equidistant. The fuel amount needed to complete the track is 100 and the canister amounts are 26, 51, 12 and 11. Staring at 26 let's the car finish; starting at 51 doesn't. In same cases, of course, there are multiple valid starting points. When reducing, it doesn't matter whether you start with 26 or 51—you still know it can be done, but a little extra record-keeping is needed to determine where to start.
I do think the problem should have explicitly stated that the fuel was not distributed equally among all canisters. If you argue that I should, in my solution, not include restrictions not imposed by the problem statement, then I will simply have the driver walk to the nearest canister, bring it back to the car and repeat as necessary. Problem solved! If you argue that I shouldn't make unreasonable assumptions, then I would respond that reasonable is a subjective term; if something is important to the problem, it should be included. Personally, I don't think it would hurt to include both.
Thanks for taking the time to create this puzzle, though (assuming it's yours).