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A277977
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a(n) = n*(1-3n+2*n^2+2*n^3)/2.
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0
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0, 1, 19, 96, 298, 715, 1461, 2674, 4516, 7173, 10855, 15796, 22254, 30511, 40873, 53670, 69256, 88009, 110331, 136648, 167410, 203091, 244189, 291226, 344748, 405325, 473551, 550044, 635446, 730423, 835665, 951886, 1079824, 1220241, 1373923, 1541680, 1724346
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OFFSET
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0,3
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COMMENTS
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For n>=3, a(n) is the second Zagreb index of the graph obtained by joining one vertex of a complete graph K[n] with each vertex of a second complete graph K[n].
The second Zagreb index of a simple connected graph g is the sum of the degree products d(i)d(j) over all edges ij of g.
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LINKS
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FORMULA
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G.f.: x*(1+x)*(1+13*x-2*x^2)/(1-x)^5. - Robert Israel, Nov 07 2016
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EXAMPLE
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a(4) = 298. Indeed, the corresponding graph has 16 edges. We list the degrees of their endpoints: (3,3), (3,3), (3,3), (3,7), (3,7), (3,7), (4,4), (4,4), (4,4), (4,4), (4,4), (4,4), (4,7), (4,7), (4,7), (4,7). Then, the second Zagreb index is 3*9 + 3*21 + 6*16 + 4*28 = 298.
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MAPLE
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seq((1/2)*n*(1-3*n+2*n^2+2*n^3), n = 0 .. 45);
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PROG
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(PARI) concat(0, Vec(x*(1+x)*(1+13*x-2*x^2)/(1-x)^5 + O(x^40))) \\ Felix Fröhlich, Nov 07 2016
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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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