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A350043
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Sum of all the parts > 1 in the partitions of n into 3 positive integer parts.
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1
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0, 2, 7, 15, 24, 36, 58, 75, 104, 138, 175, 217, 277, 328, 399, 477, 560, 650, 766, 869, 1000, 1140, 1287, 1443, 1633, 1806, 2015, 2235, 2464, 2704, 2986, 3247, 3552, 3870, 4199, 4541, 4933, 5300, 5719, 6153, 6600, 7062, 7582, 8073, 8624, 9192, 9775, 10375, 11041, 11674
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OFFSET
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3,2
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LINKS
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Index entries for linear recurrences with constant coefficients, signature (0,2,2,-1,-4,-1,2,2,0,-1).
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FORMULA
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For n >= 4, a(n) = -1 - floor((n-1)/2) + n * Sum_{k=1..floor(n/3)} floor((n-3*k+2)/2).
G.f.: -x^4 * (x^9-x^8-3*x^7+5*x^5+6*x^4-6*x^3-11*x^2-7*x-2) / ((x+1)^2 *(x^2+x+1)^2 *(x-1)^4). - Alois P. Heinz, Dec 13 2021
a(n) = 2*a(n-2)+2*a(n-3)-a(n-4)-4*a(n-5)-a(n-6)+2*a(n-7)+2*a(n-8)-a(n-10). - Wesley Ivan Hurt, Dec 17 2021
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EXAMPLE
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a(7) = 24; The partitions of 7 into 3 positive integer parts are (1,1,5), (1,2,4), (1,3,3) and (2,2,3). The sum of all the parts > 1 is then 5+2+4+3+3+2+2+3 = 24.
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MATHEMATICA
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CoefficientList[Series[-x*(x^9 - x^8 - 3*x^7 + 5*x^5 + 6*x^4 - 6*x^3 - 11*x^2 - 7*x - 2)/((x + 1)^2*(x^2 + x + 1)^2*(x - 1)^4), {x, 0, 50}], x] (* Wesley Ivan Hurt, Nov 12 2022 *)
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PROG
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(PARI) a(n)=if(n==3, 0, -1 - floor((n-1)/2) + n * sum(k=1, floor(n/3), floor((n-3*k+2)/2))) \\ Winston de Greef, Jan 28 2024
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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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