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A051938
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Truncated triangular numbers: a(n) = n*(n+1)/2 - 18.
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5
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3, 10, 18, 27, 37, 48, 60, 73, 87, 102, 118, 135, 153, 172, 192, 213, 235, 258, 282, 307, 333, 360, 388, 417, 447, 478, 510, 543, 577, 612, 648, 685, 723, 762, 802, 843, 885, 928, 972, 1017, 1063, 1110, 1158, 1207, 1257, 1308, 1360, 1413, 1467, 1522, 1578
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OFFSET
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6,1
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COMMENTS
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If a 3-set Y and a 3-set Z, having one element in common, are subsets of an n-set X then a(n+2) is the number of 3-subsets of X intersecting both Y and Z. - Milan Janjic, Oct 03 2007
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LINKS
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FORMULA
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G.f.: x^6*(3*x^2-x-3) / (x-1)^3. - Colin Barker, Mar 18 2015
Sum_{n>=6} 1/a(n) = 4423/6120 + 2*Pi*tan(sqrt(145)*Pi/2)/sqrt(145). - Amiram Eldar, Dec 13 2022
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MATHEMATICA
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PROG
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(PARI) Vec(x^6*(3*x^2-x-3)/(x-1)^3 + O(x^100)) \\ Colin Barker, Mar 18 2015
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 21 1999
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STATUS
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approved
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