The hops cross once
Worth reading first: The organ that was taken away · Where a handover sits.
A lattice’s two contact families are the two lags whose hop across the surface is shortest. A handover is a rise at which they change places. Every rung of the ladder was searched for one, and six of the ladder’s rungs have one.
That search swept the ladder at a ratio — a constant step in the logarithm of the rise, which is the right instrument for finding rungs across two decades and is coarser than the grid the rises are named on. It reports one handover per rung and nobody had asked whether that is because there is one or because the sweep found the first.
What it costs to ask
Almost nothing, and that is the whole reason this exists. A hop length is closed-form geometry: the settled divergence gives the angle between consecutive organs, and the hop of a lag follows from the angle and the rise without growing anything.
One stem a rise to find the divergence, and then forty hop lengths evaluated and sorted. The Lucas 7/11 rung holds 378 rises at the hundred-thousandth grid, and walking all of them takes about three minutes.
That is two orders of magnitude cheaper than any question that needs a cut, and it is the reason a rung can be walked where a band has to be sampled.
The answer
Exactly one crossing, between h = 0.00804 and h = 0.00803. Above it the 7-hop is shorter; below it the 11-hop is, at every one of the remaining rises down to the rung’s fine end at 0.00570.
So this rung has one handover. That is the answer the coarser sweep gave and it is now a measurement rather than the first thing found.
It is also the answer that makes a band meaningful. A band is grown outwards from a handover, and a rung with two would have two bands and no reason to prefer either.
The ladder records it somewhere else
At 0.00800, which is neither of the two rises the fine walk brackets the crossing between.
That is not an error. The ladder sweep steps by a ratio, and near the fine end of the Lucas branch that step is about half a per cent of the rise, so it visits 0.00800 and does not visit 0.00804 or 0.00803. It reports the rise it visited that is nearest the crossing.
The difference is 0.44 per cent of the rise, and nothing built on the handover moves: the band grown around it is 124 rises wide and the four-thousandths of a per cent of a rung in question is inside it many times over.
Which is worth writing down anyway
Because the handover has been used as a rise in comparisons, and a rise used in a comparison should be a rise somebody has located.
The comparison it is used in is the one this thread rests on: whether the rise where the survivor changes is the rise where the hops cross. Until now the second of those was known to a sweep step and the first to a bracket, and neither was known to the grid.
Now both are known to one step of the grid on this rung, and the comparison is between two located numbers rather than between two approximations.
How near the two hops come
At the crossing the second-shortest hop is longer than the shortest by a factor of 1.0003, which is three parts in ten thousand. That is what a crossing looks like when it is sampled at a grid: the two lengths never come out equal, they come out as near as the grid allows.
Away from the crossing the ratio rises to 1.104 at the rung’s ends. So the ordering is decisive over most of the rung and marginal over a handful of rises around the crossing.
That has a consequence for anything read near a handover. A quantity that depends on which hop is shorter is reading a distinction of three parts in ten thousand there, and a quantity that depends on how much shorter is reading nearly nothing.
The separation rule
The counter that returns a contact pair declines to order the two steps when they are within a stated ratio of each other, and the rule is not a convenience: two steps that differ by less than the grid can resolve are two steps whose ordering is a property of the grid.
On this rung the ordering is refused at the crossing and nowhere else, which is what a single crossing on a smooth pair of curves should produce.
A rung where the refusal covered a stretch would be a rung whose handover is not a rise but an interval, and none of the six is like that.
Why one crossing is not obvious
The two hop lengths are not monotone in the rise. Each is the distance across a cylinder between organs a fixed number apart, and as the rise falls the whole lattice compresses in one direction while the divergence drifts, so both curves bend.
Two bending curves can cross any number of times. What the walk shows is that on this rung they cross once, and it shows it by evaluating both at every rise rather than by an argument.
The same walk on the other five rungs is the obvious extension and has not been run. It is three minutes a rung, and the six bands built around those handovers are all built on the assumption this walk has now tested on one of them.
What a second crossing would have meant
That the band built around the recorded handover is one of two, chosen by which the ladder sweep found first — and that every claim about the handover on that rung is a claim about an arbitrary one of them.
It would also have meant something about the counted pair. A rung is a stretch over which one pair is returned, and a pair whose two members exchange the shortest hop twice inside one rung is a stranger object than one that exchanges once.
Neither is the case here. The result is a negative and it is the negative that lets everything else stand.
The rung’s own extent
From 0.00947 to 0.00570, which is a factor of 1.66 in the rise and 0.508 in the logarithm. The crossing at 0.008035 sits 33 per cent of the way down it from the coarse end.
That is the quantity a handover’s position inside its rung is measured by, and across the six rungs with handovers it runs from 5 per cent to 40. So this rung’s handover is neither central nor at an end.
Where it sits is not decided by anything anybody chose: a rung ends where the counted pair changes and the handover falls where the two hops cross, and both are read off the same geometry.
What the walk does not answer
Whether the crossing means anything to a cut. That is the question the same rung was cut for, and the two answers sit far apart: the family a cut keeps first includes 11 between 0.00640 and 0.00639, which is a hundred and sixty-four grid steps below the crossing.
So the geometry and the ablation disagree about where anything happens on this rung, which is the point of having both.
The walk’s contribution is to make that a disagreement between two located numbers rather than between a sweep step and a bracket.
Sampling a smooth quantity, for once
Everything else in this thread is discrete: a surviving family is an integer, a cut wrecks or does not, an offset is in the wrecking set or is not. Those quantities change at single rises and can only be found by looking at every rise.
Hop lengths are not like that. They are smooth functions of the rise, they can be interpolated, and the crossing could have been found by bisection in a dozen evaluations rather than 378.
The walk was run anyway, because bisection assumes one crossing and one crossing was the thing being tested. Having the whole curve is also what makes the ratio at the ends available, which bisection would not have produced.
What the ratio at the ends is worth
It bounds how much of the rung is near the crossing. At 1.104 at the ends and 1.0003 at the crossing, the ratio is above 1.05 over about two thirds of the rung.
So a claim of the form this rung’s cuts keep the shorter hop is a claim about a well-ordered pair over most of the rung and about a marginal one near the middle. The census cuts this rung at 0.008, which is one grid step from the crossing and therefore in the marginal region.
That is worth knowing about the census row rather than about the rung: the lattice the census holds here is the one whose two contact steps are most nearly tied on the whole ladder.
The refusal built into the walk
A rise the counter cannot read is recorded as a refusal on its own row rather than allowed to end the walk. A walk that stopped at its first unreadable rise would report where the reading fails and not where the crossing is.
On this rung nothing refuses. All 378 rises return a pair and an ordering, which is what a rung means — the pair is constant across it by construction.
The provision is there because the same machinery is used on stretches below the ladder’s finest rung, where refusals are common and are the informative part.
Five rungs unwalked
The golden 3/5, 5/8 and 8/13 and the Lucas 3/4 and 4/7 each have a recorded handover and none has been walked at the grid.
Three minutes each. What it would buy is the same two things it bought here: a crossing count per rung, and a located rise to compare anything against.
The crossing count is the interesting half. One rung with two crossings would change what a band is on that rung, and there is no argument available that says there is not one.
Why a rung is swept at a ratio in the first place
The ladder runs from a rise of 0.07 to 0.003, which is a factor of twenty-three. A fixed step in the rise is a step of about one per cent at the coarse end and a step of about thirty per cent at the fine end, so a sweep by fixed step either spends most of its budget where nothing changes or steps clean over whole rungs at the bottom.
Sweeping by ratio makes every step the same size in the quantity that matters, which is the argument the ladder sweep states for itself and the argument a band’s own step follows.
The cost is that a ratio step lands where it lands. It never visits the five-decimal grid except by coincidence, so a rise it reports is a rise it sampled and not a rise anybody named. That is the whole of the discrepancy here.
What the five-decimal grid is
Every rise on this site is rounded to five decimal places before anything is grown from it. That is not a display convention; it is the set of rises that exist.
The reason is reproducibility. A stem grown at 0.0064000001 and one grown at 0.0064 are two different runs, and a table that quotes the first is a table nobody can reproduce from what it prints. Rounding at the point of use makes every rise in every file a rise anybody can type.
It also sets the finest resolution any claim here can have. A transition located to one step of that grid is located as finely as this collection can locate anything, and asking for more would mean changing what a rise is.
What the walk says about the census’s own rise
The census cuts this rung at 0.008, which is four steps of the grid from the crossing. Of the ten lattices the census holds, that is the one nearest a handover by a wide margin.
Whether that matters is not something this walk can say, and it is worth stating as an open question rather than as a caveat. The claims the census makes are about which family a cut keeps, and the family it keeps is not the shorter hop — so a row where the two hops are nearly tied is a row where the rejected account has least to lose.
If anything it is the useful row. An account that says the cut keeps the shorter hop makes its weakest prediction where the two are tied, and the census’s nearest-to-tied row still refuses it.
Two routes to one number, again
The crossing can be computed two ways and both are in the walk. The ordering flips between two rises: at 0.00804 the shortest lag is 7 and at 0.00803 it is 11. The ratio between the two shortest lengths dips to its minimum at the same pair of rises.
Those are the same fact read off two different quantities, and they agree — which is what the site asks of every number it reports. The ratio’s minimum is the more useful of the two because it is continuous: it says not only where the crossing is but how sharp it is.
Two routes to one number is the discipline this whole collection runs on, and it is cheapest exactly here, where both routes are arithmetic on a settled divergence.
The stem behind each rise
One per rise, grown until its divergence stops changing, and the divergence is what every hop length is computed from. So the walk is not free of stems — it is free of cut stems, which is the expensive kind.
A cut needs two runs: the control and the cut run, grown to the same length with the same history below the hole. A hop length needs one run and then closed-form arithmetic, which is why 378 rises of geometry costs three minutes and twenty rises of cutting costs seven.
That ratio is what makes the design of this round possible at all. The geometry can be walked at the grid and the cuts have to be sampled, so the two quantities being compared are known to very different precisions unless the cutting is aimed at a bracket somebody has already narrowed.
What the walk is a template for
Any question about a lattice that does not need a cut. The expensive thing in this thread is growing a second stem with an organ removed and comparing it to a control; everything read off an intact stem is one run and closed-form arithmetic afterwards.
That distinction is worth keeping in view when a question is being designed. Where do the two contact steps cross is geometry and costs three minutes for a whole rung. Where does the family a cut keeps change is ablation and costs seven minutes for twenty rises. The two differ by two orders of magnitude in coverage per minute.
So a round that wants both quantities at the same resolution has to spend its cutting where the geometry has already narrowed the bracket. That is exactly the design this round used, and the walk is the half that made it affordable.
The five rungs this leaves
Each is three minutes and each could hold a second crossing. There is no argument available that says it does not — the two hop lengths are non-monotone in the rise, and two bending curves can meet any number of times.
What a second crossing would mean is worth stating in advance, because it decides how interesting the check is. It would mean that rung has two handovers, that the band grown around the recorded one is a band around whichever the ladder sweep found first, and that every claim about the handover on that rung is a claim about an arbitrary one of two.
Fifteen minutes for all five. It is the cheapest unspent check in this thread.
What is claimed
That the two contact steps of the Lucas 7/11 lattice change places exactly once across the whole rung, between 0.00804 and 0.00803, over 378 rises walked at the hundred-thousandth grid with no cut stems.
That the ladder sweep records the handover at 0.00800, which is 0.44 per cent of the rise away and is the nearest rise it visits, and that nothing built on the handover moves as a result.
And that the two hops come within three parts in ten thousand of each other there and within ten per cent at the rung’s ends, so the ordering is marginal near the crossing and decisive over most of the rung.
What links here
Computed from the collection, not written here: the essays that point at this one.
Shares its objects with
Essays that name at least two of the same things, and that neither author linked.
- Nine rises were not enough — both name claim testing, contact family, discretisation, handover, honest limits, instrument setting, negative result, resolution
- A band with nothing inside it — both name claim testing, contact family, handover, honest limits, negative result, resolution, rung
- Every rise of a band — both name claim testing, handover, honest limits, measurement, negative result, resolution, rung
- The alternation is not a period — both name claim testing, handover, honest limits, measurement, negative result, resolution, rung
- The offsets that never change — both name claim testing, handover, honest limits, measurement, negative result, resolution, rung
- The third band, cut whole — both name claim testing, contact family, handover, honest limits, negative result, resolution, rung
Named objects
A flat tag is an object no other essay names yet.
Claim testingContact familyDiscretisationGeometric ladderHandoverHonest limitsHop lengthInstrument settingMeasurementNegative resultResolutionRung