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 A309339 This sequence a(n) gives the minimum odd number of Z^3 vectors, each of magnitude square root of 2n, that together sum to the zero vector.  When no such minimum exists for a particular n, we set a(n) = 0. 0
 3, 0, 3, 3, 5, 0, 3, 0, 3, 0, 9, 3, 3, 0, 5, 3, 5, 0, 3, 5, 3, 0, 5, 0, 3, 0, 3, 3, 11, 0, 3, 0, 5, 0, 7, 3, 3, 0, 3, 0, 7, 0, 3, 9, 5, 0, 5, 3, 3, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The following observations are shown by Chow and Ionascu, respectively, in the first two referenced articles. There exists an odd number of Z^3 vectors, each of magnitude square root of z, that together sum to the zero vector if and only if the squarefree part of z is even. There exist three Z^3 vectors, each of magnitude square root of z, that together sum to the zero vector if and only if the squarefree part of z is even, but contains no odd prime factor congruent to 2 (mod 3). LINKS T. Chow, Distances forbidden by two-colorings of Q^3 and A_n, Discrete Math. Vol. 115 (1993), 95-102. E. J. Ionascu, A parameterization of equilateral triangles having integer coordinates, J. Integer Sequences Vol. 10 (2007), #07.6.7. E. J. Ionascu, Counting all equilateral triangles in {0, 1, ..., n}, Acta Math. Univ. Comenianae Vol. 77 (1) (2008), 129-140. EXAMPLE As an example, a(1) = 3 as evidenced by the vectors (1,0,1), (0,1,-1), and (-1,-1,0). We have a(5) > 3 by Ionascu's result, and to see that a(5) = 5, consider the vectors (-3,1,0), (1,0,3), (1,0,3), (1,0,-3), and (0,-1,-3). CROSSREFS Sequence in context: A261922 A078907 A282135 * A333453 A278923 A210485 Adjacent sequences:  A309336 A309337 A309338 * A309340 A309341 A309342 KEYWORD nonn,more AUTHOR Matt Noble, Jul 24 2019 STATUS approved

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Last modified June 20 06:56 EDT 2021. Contains 345157 sequences. (Running on oeis4.)