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A002789
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Number of integer points in a certain quadrilateral scaled by a factor of n.
(Formerly M1039 N0390)
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5
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2, 4, 7, 11, 16, 21, 28, 35, 43, 52, 62, 72, 84, 96, 109, 123, 138, 153, 170, 187, 205, 224, 244, 264, 286, 308, 331, 355, 380, 405, 432, 459, 487, 516, 546, 576, 608, 640, 673, 707, 742, 777, 814, 851, 889, 928, 968, 1008, 1050, 1092, 1135, 1179, 1224, 1269
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OFFSET
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1,1
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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 closed line segment from (1/2, 1/3) to (0, 1) is not included but the rest of the boundary is. The sequence is denoted by d(n). - Michael Somos, May 22 2014
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REFERENCES
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N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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G.f.: x * (2 + 2*x + x^2) / (1 - x - x^2 + x^4 + x^5 - x^6) = (2*x + x^3 + x^4 + x^5) / ((1 - x)^2 * (1 - x^6)). - Michael Somos, May 22 2014
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EXAMPLE
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G.f. = 2*x + 4*x^2 + 7*x^3 + 11*x^4 + 16*x^5 + 21*x^6 + 28*x^7 + 35*x^8 + ...
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MATHEMATICA
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a[ n_] := Quotient[ 7 + 12 n + 5 n^2, 12]; (* Michael Somos, May 22 2014 *)
a[ n_] := Length @ With[{o = Boole[ 0 < n], c = Boole[ 0 >= n], m = Abs@n}, FindInstance[ 0 < o + x && 0 < o + y && (2 x < o + m && 4 x + 3 y < c + 3 m || m < c + 2 x && 2 x + 3 y < o + 2 m), {x, y}, Integers, 10^9]]; (* Michael Somos, May 22 2014 *)
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PROG
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(PARI) {a(n) = (7 + 12*n + 5*n^2) \ 12}; /* Michael Somos, May 22 2014 */
(PARI) {a(n) = if( n<0, polcoeff( x^3 * (1 + x + x^2 + 2*x^4) / ((1 - x)^2 * (1 - x^6)) + x * O(x^-n), -n), polcoeff( x * (2 + x^2 + x^3 + x^4) / ((1 - x)^2 * (1 - x^6)) + x * O(x^n), n))}; /* Michael Somos, May 22 2014 */
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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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