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A112087
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a(n) = 4*(n^2 - n + 1).
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4
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4, 12, 28, 52, 84, 124, 172, 228, 292, 364, 444, 532, 628, 732, 844, 964, 1092, 1228, 1372, 1524, 1684, 1852, 2028, 2212, 2404, 2604, 2812, 3028, 3252, 3484, 3724, 3972, 4228, 4492, 4764, 5044, 5332, 5628, 5932, 6244, 6564, 6892, 7228, 7572, 7924, 8284
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OFFSET
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1,1
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COMMENTS
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Diagonal of a number spiral.
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = tanh(Pi*sqrt(3)/2)*Pi/(4*sqrt(3)). - Amiram Eldar, Aug 25 2022
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EXAMPLE
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..43.44.45.46.47.48.49.50...
..42.21.22.23.24.25.26.51...
..40.19..7..8..9.10.27.52...
..40.19..6..1..2.11.28....
..39.18..5..4..3.12.29
..38.17.16.15.14.13.30
..37.36.35.34.33.32.31
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MATHEMATICA
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PROG
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(Maxima) makelist(4*n^2 - 4*n + 4, n, 1, 55); /* Martin Ettl, Oct 16 2012 */
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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Jenny X. Chen (pookahead(AT)gmail.com), Nov 28 2005
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EXTENSIONS
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STATUS
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approved
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