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A304619
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a(n) = 324*n^2 - 804*n + 468 (n>=2).
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2
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156, 972, 2436, 4548, 7308, 10716, 14772, 19476, 24828, 30828, 37476, 44772, 52716, 61308, 70548, 80436, 90972, 102156, 113988, 126468, 139596, 153372, 167796, 182868, 198588, 214956, 231972, 249636, 267948, 286908, 306516, 326772, 347676, 369228, 391428, 414276, 437772, 461916, 486708
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OFFSET
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2,1
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COMMENTS
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For n>=3, a(n) is the second Zagreb index of the hexagonal network HX(n).
The second Zagreb index of a simple connected graph is the sum of the degree products d(i)d(j) over all edges ij of the graph.
The M-polynomial of the hexagonal network HX(n) is M(HX(n); x,y) = 12*x^3*y^4 + 6*x^3*y^6 + 6*(n-3)*x^4*y^4 + 12*(n-2)*x^4*y^6 + (9*n^2-33*n+30)*x^6*y^6.
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LINKS
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B. Rajan , A. William, C. Grigorius, and S. Stephen, On certain topological indices of silicate, honeycomb and hexagonal networks, J. Comp. & Math. Sci., 3, No. 5, 2012, 530-535.
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FORMULA
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G.f.: 12*x^2*(13 + 42*x - x^2) / (1 - x)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n>4.
(End)
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MAPLE
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seq(468-804*n+324*n^2, n = 2..40);
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PROG
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(GAP) List([2..40], n->324*n^2-804*n+468); # Muniru A Asiru, May 18 2018
(PARI) a(n) = 324*n^2 - 804*n + 468; \\ Altug Alkan, May 18 2018
(PARI) Vec(12*x^2*(13 + 42*x - x^2) / (1 - x)^3 + O(x^40)) \\ Colin Barker, May 18 2018
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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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