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May 20, 2015 at 7:23 comment added kodlu Actually, if you don't require constant Hamming distance, you can double the size of the Walsh-Hadamard code by allowing the binary complement of each codeword to be also a codeword.
Sep 12, 2011 at 15:52 comment added Emil Jeřábek @Gil: Thanks for editing it in the answer.
Sep 9, 2011 at 22:12 history edited Gil Kalai CC BY-SA 3.0
Added additional material from the comments.; deleted 1 characters in body
Sep 9, 2011 at 18:16 comment added Emil Jeřábek That’s a good point. Moreover, the conjecture holds for Jean-Michel’s $n=80$: one can take the tensor product of an order 4 Hadamard matrix as above with an order $2(9+1)$ Paley construction II as explained in en.wikipedia.org/wiki/Paley_construction . So the answer to the original question is 80.
Sep 9, 2011 at 17:44 comment added Gil Kalai Indeed it is a famous conjecture that such codes exist whenever n =0 (mod 4). It is easy to see that this is necessary. See en.wikipedia.org/wiki/Hadamard_matrix
Sep 9, 2011 at 15:53 comment added Emil Jeřábek I should also mention that the construction for $n=2^k$ is know as the Walsh–Hadamard code en.wikipedia.org/wiki/Walsh%E2%80%93Hadamard_code .
Sep 9, 2011 at 15:51 vote accept Jean-Michel
Sep 9, 2011 at 15:34 history answered Emil Jeřábek CC BY-SA 3.0