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A231589
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a(n) = sum_{k=1..(n-1)/2} (k^2 mod n).
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3
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0, 0, 1, 1, 5, 5, 7, 6, 12, 20, 22, 19, 39, 35, 35, 28, 68, 60, 76, 65, 91, 99, 92, 74, 125, 156, 144, 147, 203, 175, 186, 152, 242, 272, 245, 201, 333, 323, 286, 270, 410, 392, 430, 363, 420, 437, 423, 340, 490, 550, 578, 585, 689, 639, 605, 546, 760, 812
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OFFSET
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1,5
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COMMENTS
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This expression occurred to S. A. Shirali while demonstrating a result concerning A081115 and A228432. This led him to investigate integers n such that a(n) = n*(n-1)/4, a(n) = floor(n/4), or a(n) = n*(n-1)/4 - n.
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LINKS
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MATHEMATICA
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Table[Sum[PowerMod[k, 2, n], {k, (n-1)/2}], {n, 60}] (* Harvey P. Dale, Jan 30 2016 *)
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PROG
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(PARI) a(n) = sum(k=1, (n-1)\2, k^2 % n);
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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