Abstract
Symmetric nets are affine resolvable designs whose duals are also affine. It is shown that. up to isomorphism, there are exactly four symmetric (3, 3)-nets (v=b=27,k=9), and exactly two inequivalent 9×9 generalized Hadamard matrices over the group of order 3. The symmetric (3, 3)-nets are found as subnets of affine resolvable 2-(27, 9, 4) designs. Ten of the 68 non-isomorphic affine resolvable 2-(27, 9, 4) designs are not extensions of symmetric (3, 3)-subnets, providing the first examples of affine 2-(q3, q2, q2−1/q−1) designs without symmetric (q, q)-subnets.
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Mavron, V.C., Tonchev, V.D. On symmetric nets and generalized Hadamard matrices from affine designs. J Geom 67, 180–187 (2000). https://doi.org/10.1007/BF01220309
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DOI: https://doi.org/10.1007/BF01220309