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A304380
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a(n) = 36*n^2 - 4*n (n>=1).
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3
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32, 136, 312, 560, 880, 1272, 1736, 2272, 2880, 3560, 4312, 5136, 6032, 7000, 8040, 9152, 10336, 11592, 12920, 14320, 15792, 17336, 18952, 20640, 22400, 24232, 26136, 28112, 30160, 32280, 34472, 36736, 39072, 41480, 43960, 46512, 49136, 51832, 54600, 57440
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OFFSET
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1,1
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COMMENTS
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a(n) is the first Zagreb index of the octagonal network O(n,n); O(m,n) is defined by Fig. 1 of the Siddiqui et al. reference.
The first Zagreb index of a simple connected graph is the sum of the squared degrees of its vertices. Alternatively, it is the sum of the degree sums d(i) + d(j) over all edges ij of the graph.
The M-polynomial of O(n,n) is M(O(n,n);x,y) = 4*(n+1)x^2*y^2 + 8(n-1)x^2 *y^3 + (6n^2 - 10n+4)x^3*y^3.
More generally, the M-polynomial of O(m,n) is M(O(m,n); x,y) = 2(m+n+2)x^2*y^2+4(m+n-2)x^2 *y^3+(6mn-5m-5n+4)x^3*y^3.
Sequence found by reading the line from 32, in the direction 32, 136, ..., in the square spiral whose vertices are the generalized 20-gonal numbers. - Omar E. Pol, May 13 2018
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LINKS
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FORMULA
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G.f.: 8*x*(4 + 5*x) / (1 - x)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n>3.
(End)
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MAPLE
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seq(36*n^2-4*n, n = 1 .. 40);
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PROG
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(PARI) Vec(8*x*(4 + 5*x) / (1 - x)^3 + O(x^40)) \\ Colin Barker, May 13 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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