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A112742
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a(n) = n^2*(n^2 - 1)/3.
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8
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0, 0, 4, 24, 80, 200, 420, 784, 1344, 2160, 3300, 4840, 6864, 9464, 12740, 16800, 21760, 27744, 34884, 43320, 53200, 64680, 77924, 93104, 110400, 130000, 152100, 176904, 204624, 235480, 269700, 307520, 349184, 394944, 445060, 499800, 559440
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OFFSET
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0,3
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COMMENTS
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Second derivative of the n-th Chebyshev polynomial (of the first kind) evaluated at x=1.
The second derivative at x=-1 is just (-1)^n * a(n).
The difference between two consecutive terms generates the sequence a(n+1) - a(n) = A002492(n).
Consider the partitions of 2n into two parts (p,q) where p <= q. Then a(n) is the total volume of the family of rectangular prisms with dimensions p, |q-p| and |q-p|. - Wesley Ivan Hurt, Apr 15 2018
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LINKS
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FORMULA
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a(n) = (n-1)*n^2*(n+1)/3 = 4*A002415(n).
a(n) = 5*a(n-1) - 10*a(n-2) + 10*a(n-3) - 5*a(n-4) + a(n-5).
G.f.: 4*x^2*(1+x)/(1-x)^5. (End)
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EXAMPLE
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a(4)=80 because
C_4(x) = 1 - 8x^2 + 8x^4,
C'_4(x) = -16x + 32x^3,
C''_4(x) = -16 + 96x^2,
C''_4(1) = -16 + 96 = 80.
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MATHEMATICA
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Table[D[ChebyshevT[n, x], {x, 2}], {n, 0, 100}] /. x -> 1
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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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Matthew T. Cornick (maruth(AT)gmail.com), Sep 16 2005
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STATUS
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approved
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