Even Sets and Dual Projective Geometric Codes: A Tale of Cylinders
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arXiv
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| Format: | Preprint |
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2026
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| _version_ | 1866917209442353152 |
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| author | Adriaensen, Sam |
| author_facet | Adriaensen, Sam |
| contents | In this paper, we prove that the smallest even sets in ${\rm PG}(n,q)$, i.e. sets that intersect every line in an even number of points, are cylinders with a hyperoval as base. This fits into a more general study of dual projective geometric codes. Let $q$ be a prime power, and define $\mathcal C_k(n,q)^\perp$ as the kernel of the $k$-space vs. point incidence matrix of ${\rm PG}(n,q)$, seen as a matrix over the prime order subfield of $\mathbb F_q$. Determining the minimum weight of this linear code is still an open problem in general, but has been reduced to the case $k=1$. There is a known construction that constructs small weight codewords of $\mathcal C_1(n,q)^\perp$ from minimum weight codewords of $\mathcal C_1(2,q)^\perp$. We call such codewords cylinder codewords. We pose the conjecture that all minimum weight codewords of $\mathcal C_1(n,q)^\perp$ are cylinder codewords. This conjecture is known to be true if $q$ is prime. We take three steps towards proving that the conjecture is true in general:
(1) We prove that the conjecture is true if $q$ is even. This is equivalent to our classification of the smallest even sets.
(2) We prove that the minimum weight of $\mathcal C_1(n,q)^\perp$ is $q^{n-2}$ times the minimum weight of $\mathcal C_1(2,q)^\perp$, which matches the weight of cylinder codewords. Thus, we completely reduce the problem of determining the minimum weight of $\mathcal C_1(n,q)^\perp$ to the case $n=2$.
(3) We prove that if the conjecture is true for $n=3$, it is true in general. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_12451 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Even Sets and Dual Projective Geometric Codes: A Tale of Cylinders Adriaensen, Sam Combinatorics 51E20, 94B25, 05B25 In this paper, we prove that the smallest even sets in ${\rm PG}(n,q)$, i.e. sets that intersect every line in an even number of points, are cylinders with a hyperoval as base. This fits into a more general study of dual projective geometric codes. Let $q$ be a prime power, and define $\mathcal C_k(n,q)^\perp$ as the kernel of the $k$-space vs. point incidence matrix of ${\rm PG}(n,q)$, seen as a matrix over the prime order subfield of $\mathbb F_q$. Determining the minimum weight of this linear code is still an open problem in general, but has been reduced to the case $k=1$. There is a known construction that constructs small weight codewords of $\mathcal C_1(n,q)^\perp$ from minimum weight codewords of $\mathcal C_1(2,q)^\perp$. We call such codewords cylinder codewords. We pose the conjecture that all minimum weight codewords of $\mathcal C_1(n,q)^\perp$ are cylinder codewords. This conjecture is known to be true if $q$ is prime. We take three steps towards proving that the conjecture is true in general: (1) We prove that the conjecture is true if $q$ is even. This is equivalent to our classification of the smallest even sets. (2) We prove that the minimum weight of $\mathcal C_1(n,q)^\perp$ is $q^{n-2}$ times the minimum weight of $\mathcal C_1(2,q)^\perp$, which matches the weight of cylinder codewords. Thus, we completely reduce the problem of determining the minimum weight of $\mathcal C_1(n,q)^\perp$ to the case $n=2$. (3) We prove that if the conjecture is true for $n=3$, it is true in general. |
| title | Even Sets and Dual Projective Geometric Codes: A Tale of Cylinders |
| topic | Combinatorics 51E20, 94B25, 05B25 |
| url | https://arxiv.org/abs/2601.12451 |