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A028569
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a(n) = n*(n + 9).
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23
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0, 10, 22, 36, 52, 70, 90, 112, 136, 162, 190, 220, 252, 286, 322, 360, 400, 442, 486, 532, 580, 630, 682, 736, 792, 850, 910, 972, 1036, 1102, 1170, 1240, 1312, 1386, 1462, 1540, 1620, 1702, 1786, 1872, 1960, 2050, 2142, 2236, 2332, 2430
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OFFSET
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0,2
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COMMENTS
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a(n) is the first Zagreb index of the wheel graph with n + 1 vertices. The first Zagreb index of a simple connected graph is the sum of the squared degrees of its vertices. Alternately, it is the sum of the degree sums d(i) + d(j) over all edges ij of the graph. - Emeric Deutsch, Nov 07 2016
The sequence provides all nonnegative k such that 4*k + 81 is a square. - Bruno Berselli, May 08 2018
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LINKS
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FORMULA
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Sum_{n >= 1} 1/a(n) = 7129/22680 = 0.314329806... - R. J. Mathar, Mar 22 2011
Sum_{n>=1} (-1)^(n+1)/a(n) = 2*log(2)/9 - 1879/22680. - Amiram Eldar, Jan 15 2021
Product_{n>=1} (1 - 1/a(n)) = -128*cos(sqrt(85)*Pi/2)/(19*Pi).
Product_{n>=1} (1 + 1/a(n)) = 51840*cos(sqrt(77)*Pi/2)/(4199*Pi). (End)
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MAPLE
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MATHEMATICA
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PROG
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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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STATUS
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approved
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