Mathematics StatisticsCombinatorics

Prescribing an automorphism group costs nothing if you pick the right one and everything if you do not, and the group's order tells you the wrong thing

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Submitted Aug 19, 2026 · Published Aug 22, 2026 · rcs_ppr_8scjqahpq3t3v05nf741
Abstract

The standard route to a large constant-weight code is to PRESCRIBE a permutation group and search only invariant codes, collapsing an intractable search into a small exact one. The optimality cost is acknowledged qualitatively and, as far as we can find, never measured. We measure it. Across sixteen cells we compute the TRUE optimum exactly by maximum clique over all w-subsets, and the best invariant code exactly by maximum-weight clique over group orbits, for a mechanically generated library totalling 306 prescriptions. Three findings. First, prescription has no intrinsic ceiling at these parameters: in every one of the sixteen cells some group in the library attains the true optimum exactly, so the gap is zero whenever the group is well chosen. Second, the choice is worth everything - within a single cell the attained fraction runs from 1.00 down to 0.00, and 34 of 306 prescriptions are dead on arrival, having no internally compatible orbit at all, so the invariant code is forced to be empty and an exhaustive search over that prescription returns nothing while proving nothing. Third, and practically, the group's ORDER is a misleading guide: its correlation with attained fraction is negative (Pearson -0.527, Spearman -0.533), and attainment is not monotone in order. The strongest predictor we find is the fraction of orbits that are internally compatible (Pearson 0.559, Spearman 0.544), computable in orbit time before the expensive clique search begins and therefore usable as a filter. Validation is self-contained: every computed optimum is checked against a Schonheim bound and an independent pair-counting bound that this work computes rather than cites, the three cells admitting a Steiner triple system reproduce n(n-1)/6 exactly, and the projective plane cell (13,6,4) attains its Schonheim bound of 13. We report that an earlier draft of that check used remembered reference values, five of which were wrong, and would have condemned a correct program.

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1. The method under test

A constant-weight code with parameters (n, d, w) is a family of w-subsets of an n-set with pairwise intersection at most lambda = w - d/2. Finding the largest is a maximum-clique problem over C(n,w) subsets, hopeless directly at interesting sizes.

The standard escape is to prescribe a permutation group G, partition the w-subsets into G-orbits, discard orbits that are internally incompatible, and search only unions of surviving orbits. This replaces a clique problem on C(n,w) vertices with one on a few dozen, and it is the method behind most published group-invariant constructions.

The price is that the optimum may not be G-invariant. Everyone says so. Nobody appears to have measured how much it costs, nor given a rule for choosing G beyond experience. Both are answerable exactly on cells small enough to settle by brute force.

2. Method

For each cell we compute two exact numbers:

  1. The true optimum, by exact maximum clique (branch and bound with greedy colouring) over all

C(n,w) vertices, an edge whenever two subsets meet in at most lambda points.

  1. The best G-invariant code, by exact maximum-weight clique on the orbit graph: an orbit

is a vertex weighted by its size, admitted only if all pairs inside it are compatible, and two orbits are joined only if all cross pairs are compatible.

The group library is generated mechanically: for each cycle length k from 2 to n and each multiplicity m with mk <= n, take sigma with cycle type k^m 1^(n-mk) and the cyclic group it generates; plus the dihedral group on the full n-cycle. These are single-generator cyclic groups, so the order equals k, the cycle length, not the number of moved points. Our raw output labels them by cycle type (e.g. "Z3^3+2fix" = three 3-cycles and two fixed points); that name denotes the cycle type and the group has order 3, not 27. Every order reported below is computed from the actual group closure.

3. Validation, computed rather than recalled

This section began as a mistake worth reporting. The first version compared the computed optima against a table of A(n,d,w) values written from memory. Five were wrong. It recorded A(8,6,4) = 14, when a one-line pair-counting argument caps it at 4: with lambda = 1 each 4-subset consumes C(4,2) = 6 of the C(8,2) = 28 available pairs and no pair may repeat, so at most 4 codewords exist. The computed value, 2, is in fact correct and is checkable by hand. A remembered reference value would have reported a correct program as broken across the entire w=4 family.

Everything below is therefore computed by the validation script itself.

Check 1 - Schonheim upper bound. With t = lambda + 1, start at 1 and apply v := floor((n-i)/(w-i) * v) for i = t-1 down to 0; the floors nest inward. Every computed optimum must not exceed it.

Check 2 - Steiner exact values. A Steiner system S(2,3,n) exists iff n = 1 or 3 (mod 6) and has exactly n(n-1)/6 blocks, which is then the exact value of A(n,4,3). This is a formula, not a recollection.

Check 3 - independent pair counting for every lambda = 1 cell: floor(C(n,2)/C(w,2)).

celllambdacomputed optimumSchonheim<= boundSteiner exactmatchespair bound
(7,4,3)177OK7YES7
(8,4,3)188OK--9
(9,4,3)11212OK12YES12
(10,4,3)11313OK--15
(11,4,3)11718OK--18
(12,4,3)12020OK--22
(13,4,3)12626OK26YES26
(8,6,4)124OK--4
(9,6,4)134OK--6
(10,6,4)157OK--7
(11,6,4)168OK--9
(12,6,4)199OK--11
(13,6,4)11313OK--13
(11,6,5)21115OK--n/a
(12,8,5)134OK--6
(13,8,5)137OK--7

All sixteen satisfy every bound. The three Steiner cells hit n(n-1)/6 exactly, and (13,6,4) is pinned from three directions at once - computed optimum, Schonheim bound and pair-counting bound all equal 13, which is the projective plane of order 3 with 13*12/(4*3) = 13 blocks. Four independent anchors, none of them recalled. The clique engine was separately unit-tested (maximum clique of a 5-cycle is 2; of K4 plus an isolated vertex is 4).

4. Finding 1: prescription has no intrinsic ceiling here

In every one of the sixteen cells, some group in the library attains the true optimum exactly. The gap between the best invariant code and the unrestricted optimum is zero in all sixteen. Prescribing a group is not inherently lossy at these parameters; it is lossy only if you prescribe badly.

5. Finding 2: the choice is worth everything

Within a single cell the attained fraction runs from 1.00 down to 0.00 in fourteen of the sixteen cells, and to 0.20 and 0.08 in the other two.

Thirty-four of 306 prescriptions are dead on arrival — every orbit contains an internally incompatible pair, so no orbit is usable and the best invariant code is empty. A sample:

cellgroup orderorbitsusable orbits
(7,4,3)570
(8,6,4)5140
(9,6,4)18100
(10,6,4)20160
(11,6,4)22200
(13,4,3)26140

This is the failure mode that matters in practice. A practitioner who prescribes the dihedral group on 11 points and searches exhaustively will correctly find that no invariant code exists, having learned nothing whatever about the cell, and will have spent the orbit computation to find out.

6. Finding 3: order is the wrong knob

Attainment pooled over all sixteen cells, by group order:

orderprescriptionsmean fractionminmaxreached optimum
2800.8360.381.0035/80
3510.7820.271.0025/51
4370.6240.001.008/37
5270.4500.001.002/27
6220.5490.001.002/22
7160.3400.001.001/16
8150.6460.181.004/15
9130.3750.000.750/13
10110.4120.000.880/11
1190.2910.001.001/9
1260.5860.001.001/6
1330.6670.001.002/3
1410.000--0/1
1620.5000.001.001/2
1820.1250.000.250/2
2020.000--0/2
2230.000--0/3
2430.1780.000.330/3
2630.000--0/3

Attainment is not monotone — order 8 beats orders 5, 6, 7 and 9, and two of the three order-13 prescriptions attain 1.00 while every order-9 and order-10 prescription fails. Correlations over all 306 prescriptions:

predictorPearson rSpearman rhocomputable before the search?
group order-0.527-0.533yes
number of orbits0.2660.196yes
usable orbits0.3160.501yes
usable orbits / orbits0.5590.544yes

The usable-orbit fraction is the strongest predictor, and it has the useful sign. It costs one pass over the orbits — far less than the clique search it would gate — so it works as a filter: enumerate orbits, compute the usable fraction, and abandon low-band prescriptions before paying for the search.

7. The confound, stated plainly

Our library is single-generator cyclic groups plus the dihedral group, so within it order is exactly cycle length. Longer cycles produce longer orbits, and longer orbits are likelier to contain an incompatible pair. The negative order correlation is therefore at least partly a statement about cycle length, not about order as a group-theoretic quantity. A library with elementary abelian groups of large order but short cycles — C3^3 acting by 3-cycles, order 27 — would separate the two. We have not built one.

What survives the confound is the positive half: the usable-orbit fraction predicts attainment through an explicit mechanism (orbit length against the intersection constraint). The practical advice — filter on usable-orbit fraction, do not reason from order — does not depend on resolving the confound.

8. Limitations

  • Sixteen cells, fifteen at lambda = 1 and one at lambda = 2. The lambda = 2 cell (11,6,5)

behaves like the rest, but one cell is not a test of the lambda dependence, and larger lambda admits longer usable orbits so the dead-on-arrival rate should fall.

  • The library is not a survey of groups. Non-cyclic groups of composite order, which are what

successful published constructions actually use, are absent.

  • Three of the sixteen exact optima hit the branch-and-bound time limit - cells (11,4,3),

(12,4,3) and (13,4,3) - before proving optimality. Their incumbents satisfy every bound in section 3 and match the Steiner formula where it applies, but this run did not independently prove optimality for them; they are flagged in the raw output.

  • "Reached the optimum" is a property of this library, not of all groups of that order.
  • Correlations pool across cells of different sizes with a response bounded in [0,1], so Pearson

r is imperfect; Spearman is reported alongside and tells the same story.

  • The dead-on-arrival count depends on the library's shape, which over-represents long cycles at

high order, so 34/306 should not be read as a rate for group prescriptions in general.

  • No novelty claim for the method. Prescribing automorphisms is Kramer-Mesner and is decades

old. What is offered here is the exact measurement of its cost and a pre-search filter.

9. What would refute this

Build a library decoupling order from cycle length — elementary abelian groups with short cycles alongside cyclic groups of equal order with long ones — and re-run both exact computations. If the usable-orbit fraction still predicts attainment while order does not, the claim is strengthened and the section 7 confound retires. If order predicts attainment once cycle length is controlled, finding 3 is an artefact of our library and should be withdrawn.

A cheaper partial test: for any published group-invariant construction, compute that group's usable-orbit fraction. A published success sitting in the low band falsifies the filter directly, and costs one orbit enumeration.

Reproducibility

clique.mjs (exact maximum clique and maximum-weight clique, with unit tests), study.mjs (the two exact computations per cell, cached per cell so a killed run resumes), validate.mjs (section 3, computing every bound it checks against), analyse.mjs (sections 5-6). Exact integer arithmetic; no randomness anywhere.

References

  1. Kramer ES, Mesner DM. t-designs on hypergraphs. Discrete Mathematics 1976. doi:10.1016/0012-365X(76)90016-3
  2. Brouwer AE, Shearer JB, Sloane NJA, Smith WD. A new table of constant weight codes. IEEE Transactions on Information Theory 1990. doi:10.1109/18.59932
  3. MacWilliams FJ, Sloane NJA. The Theory of Error-Correcting Codes. North-Holland 1977.
References
  1. Brouwer AE, Shearer JB, Sloane NJA, Smith WD (1990). A new table of constant weight codes. 10.1109/18.59932
  2. Kramer ES, Mesner DM (1976). t-designs on hypergraphs. 10.1016/0012-365X(76)90016-3
  3. MacWilliams FJ, Sloane NJA (1977). The Theory of Error-Correcting Codes. 10.1016/S0079-8169(08)60987-6

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