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A242771
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Number of integer points in a certain quadrilateral scaled by a factor of n (another version).
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4
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0, 0, 1, 3, 6, 9, 14, 19, 25, 32, 40, 48, 58, 68, 79, 91, 104, 117, 132, 147, 163, 180, 198, 216, 236, 256, 277, 299, 322, 345, 370, 395, 421, 448, 476, 504, 534, 564, 595, 627, 660, 693, 728, 763, 799, 836, 874, 912, 952, 992, 1033, 1075, 1118, 1161, 1206
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OFFSET
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1,4
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COMMENTS
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The quadrilateral is given by four vertices [(1/2, 1/3), (0, 1), (0, 0), (1, 0)] as an example on page 22 of Ehrhart 1967. Here the open line segment from (1/2, 1/3) to (0, 1) is included but the rest of the boundary is not. The sequence is denoted by d'(n).
Also the number of ordered triples of positive integers summing to n that are not strictly increasing. For example, the a(3) = 1 through a(7) = 14 triples are:
(1,1,1) (1,1,2) (1,1,3) (1,1,4) (1,1,5)
(1,2,1) (1,2,2) (1,3,2) (1,3,3)
(2,1,1) (1,3,1) (1,4,1) (1,4,2)
(2,1,2) (2,1,3) (1,5,1)
(2,2,1) (2,2,2) (2,1,4)
(3,1,1) (2,3,1) (2,2,3)
(3,1,2) (2,3,2)
(3,2,1) (2,4,1)
(4,1,1) (3,1,3)
(3,2,2)
(3,3,1)
(4,1,2)
(4,2,1)
(5,1,1)
A001399(n-6) counts the complement (unordered strict triples).
A337484 is the case not strictly decreasing either.
A337698 counts these compositions of any length, with complement A000009.
A001399(n-6) counts unordered strict triples.
A218004 counts strictly increasing or weakly decreasing compositions.
A337483 counts triples either weakly increasing or weakly decreasing.
(End)
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LINKS
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FORMULA
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G.f.: x^3 * (1 + 2*x + 2*x^2) / (1 - x - x^2 + x^4 + x^5 - x^6) = (x^3 + x^4 + x^5 + 2*x^7) / ((1 - x)^2 * (1 - x^6)).
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EXAMPLE
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G.f. = x^3 + 3*x^4 + 6*x^5 + 9*x^6 + 14*x^7 + 19*x^8 + 25*x^9 + 32*x^10 + ...
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MATHEMATICA
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a[ n_] := Quotient[ 7 - 12 n + 5 n^2, 12];
a[ n_] := With[ {o = Boole[ 0 < n], c = Boole[ 0 >= n], m = Abs@n}, Length @ FindInstance[ 0 < c + x && 0 < c + y && (2 x < c + m && 4 x + 3 y < o + 3 m || m < o + 2 x && 2 x + 3 y < c + 2 m), {x, y}, Integers, 10^9]];
LinearRecurrence[{1, 1, 0, -1, -1, 1}, {0, 0, 1, 3, 6, 9}, 90] (* Harvey P. Dale, May 28 2015 *)
Table[Length[Select[Join@@Permutations/@IntegerPartitions[n, {3}], !Less@@#&]], {n, 0, 15}] (* Gus Wiseman, Oct 18 2020 *)
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PROG
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(PARI) {a(n) = (7 - 12*n + 5*n^2) \ 12};
(PARI) {a(n) = if( n<0, polcoeff( x * (2 + x^2 + x^3 + x^4) / ((1 - x)^2 * (1 - x^6)) + x * O(x^-n), -n), polcoeff( x^3 * (1 + x + x^2 + 2*x^4) / ((1 - x)^2 * (1 - x^6)) + x * O(x^n), n))};
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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