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Erschienen in: Designs, Codes and Cryptography 2-3/2019

03.09.2018

Classifying optimal binary subspace codes of length 8, constant dimension 4 and minimum distance 6

verfasst von: Daniel Heinlein, Thomas Honold, Michael Kiermaier, Sascha Kurz, Alfred Wassermann

Erschienen in: Designs, Codes and Cryptography | Ausgabe 2-3/2019

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Abstract

We determine the maximum size \(A_2(8,6;4)\) of a binary subspace code of packet length \(v=8\), minimum subspace distance \(d=6\), and constant dimension \(k=4\) to be 257. There are two isomorphism types of optimal codes. Both of them are extended LMRD codes. In finite geometry terms, the maximum number of solids in \({\text {PG}}(7,2)\) mutually intersecting in at most a point is 257. The result was obtained by combining the classification of substructures with integer linear programming techniques. This result implies that the maximum size \(A_2(8,6)\) of a binary mixed-dimension subspace code of packet length 8 and minimum subspace distance 6 is 257 as well.
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Fußnoten
1
As an example we consider \(A_2(9,6;4)\le \left\{ \left[ {\begin{matrix}{9}\\ {1}\end{matrix}}\right] _{2}A_2(8,6;3)/\left[ {\begin{matrix}{4}\\ {1}\end{matrix}}\right] _{2}\right\} _4= \left\{ \frac{17374}{15}\right\} _4\), using \(A_2(8,6;3)=34\). We have \(\left\lfloor \frac{17374}{15} \right\rfloor =1158\), \(17374-1158\cdot 15=4\), \(17374-1157\cdot 15=19\), and \(17374-1156\cdot 15=34\). Since 4 and 19 cannot be written as a non-negative linear combination of 8, 12, 14, and 15, but \(34=14+12+8\), we have \(A_2(9,6;4)\le 1156\), which improves upon the iterative Johnson bound by two. Let us remark that [19] contains an easy and fast algorithm to check the representability as non-negative integer linear combination as specified above.
 
2
Algorithmic methods taking into account known symmetries of integer linear programming formulations automatically are presented in the literature. However, we are not aware of any paper, where those approaches have been successfully applied to compute tightened upper bounds for CDCs.
 
3
Since \({\text {Stab}}_{{\text {GL}}\left( \mathbb {F}^{8}_{2}\right) }\left( \widetilde{H}\right) = \left\{ \left( {\begin{matrix}A &{} 0 \\ b &{} 1\end{matrix}}\right) \in {\text {GL}}\left( \mathbb {F}^{8}_{2}\right) \,\left| \, A \in {\text {GL}}\left( \mathbb {F}^{7}_{2}\right) \text { and } b\in \mathbb {F}_2^{7} \right. \right\} \), any point that is not incident with \(\widetilde{H}\), i.e., \(\langle (p\mid 1)\rangle \) with \(p \in \mathbb {F}_2^{7}\), can be mapped via \(\left( {\begin{matrix}I_7 &{} 0 \\ p &{} 1\end{matrix}}\right) ^{-1}\) to \(\widetilde{P}\).
 
4
We noticed that the order of the vertices makes a huge difference for the running time. For fast results, matrices with the same last row should be numbered consecutively.
 
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Metadaten
Titel
Classifying optimal binary subspace codes of length 8, constant dimension 4 and minimum distance 6
verfasst von
Daniel Heinlein
Thomas Honold
Michael Kiermaier
Sascha Kurz
Alfred Wassermann
Publikationsdatum
03.09.2018
Verlag
Springer US
Erschienen in
Designs, Codes and Cryptography / Ausgabe 2-3/2019
Print ISSN: 0925-1022
Elektronische ISSN: 1573-7586
DOI
https://doi.org/10.1007/s10623-018-0544-8

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