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A016756
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a(n) = (2*n+1)^4.
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10
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1, 81, 625, 2401, 6561, 14641, 28561, 50625, 83521, 130321, 194481, 279841, 390625, 531441, 707281, 923521, 1185921, 1500625, 1874161, 2313441, 2825761, 3418801, 4100625, 4879681, 5764801, 6765201, 7890481, 9150625, 10556001, 12117361, 13845841, 15752961, 17850625
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OFFSET
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0,2
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COMMENTS
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a(n) is the number of ordered pairs of lattice points (vectors in R^2 with integer coordinates) that are in or on a square centered at the origin with side length 2*n. - Geoffrey Critzer, Apr 20 2013
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LINKS
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FORMULA
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G.f.: (1+76*x+230*x^2+76*x^3+x^4)/(1-x)^5; see row n=5 of A060187.
E.g.f.: (1 + 80*x + 232*x^2 + 128*x^3 + 16*x^4)*exp(x); see row n=4 of A154537. (End)
Product_{n>=0} (1 + 1/a(n)) = (cos(Pi/sqrt(2)) + cosh(Pi/sqrt(2)))/2.
Product_{n>=1} (1 - 1/a(n)) = Pi*cosh(Pi/2)/8. (End)
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EXAMPLE
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a(1) = 81 because there are 9 lattice points in or on the 2 x 2 square centered at the origin, so there are 9*9 =81 ordered pairs. - Geoffrey Critzer, Apr 20 2013
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MATHEMATICA
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LinearRecurrence[{5, -10, 10, -5, 1}, {1, 81, 625, 2401, 6561}, 30] (* Harvey P. Dale, Mar 24 2020 *)
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PROG
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(PARI) vector(40, n, n--; (2*n+1)^4) \\ G. C. Greubel, Sep 15 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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