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 A317095 a(n) = 40*n. 0
 0, 40, 80, 120, 160, 200, 240, 280, 320, 360, 400, 440, 480, 520, 560, 600, 640, 680, 720, 760, 800, 840, 880, 920, 960, 1000, 1040, 1080, 1120, 1160, 1200, 1240, 1280, 1320, 1360, 1400, 1440, 1480, 1520, 1560, 1600, 1640, 1680, 1720, 1760, 1800, 1840, 1880 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) is equal to the freshwater zone below sea level for a water table of elevation n above sea level in a simplified freshwater-saltwater interface in a coastal water-table aquifer (cf. Barlow, 2003, p. 14, eq. (2) and p. 15, Fig. B-1 and B-2). From Bruno Berselli, Sep 10 2018: (Start) After 0, subsequence of A065607: 1/a(n)^2 + 1/(30*n)^2 = 1/(24*n)^2, with n > 0 and a(n) > 30*n. Also, all positive terms belong to A049094: 2^(40*n)-1 = 1024^(4*n)-1 and (25*41-1)^(4*n)-1 is divisible by 25. (End) LINKS P. M. Barlow, Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast, Circular 1262 (2003). Wikipedia, Aquifer Wikipedia, Saltwater intrusion Index entries for linear recurrences with constant coefficients, signature (2,-1). FORMULA O.g.f.: 40*x/(1 - x)^2. E.g.f.: 40*x*exp(x). - Bruno Berselli, Sep 10 2018 a(n) = 2*a(n - 1) - a(n - 2) for n > 1. - Stefano Spezia, Sep 07 2018 a(n) = A008586(A008592(n)) = 4*A008592(n). a(n) = A010692(n)*A008586(n) = 10*A008586(n). a(n) = A008602(A005843(n)) = 20*A005843(n). a(n) = A007395(n)*A008602(n) = 2*A008602(n). MATHEMATICA Table[40 n, {n, 0, 50}] (* or *) LinearRecurrence[{2, -1}, {0, 40}, 50] (* or *) CoefficientList[Series[40*x/(1 - x)^2, {x, 0, 50}], x] (* Stefano Spezia, Sep 07 2018 *) PROG (PARI) a(n) = 40*n (PARI) a(n) = if(n==0, 0, if(n==1, 40, 2*a(n-1)-a(n-2))) (PARI) concat(0, Vec(40*x/(1-x)^2 + O(x^60))) CROSSREFS Cf. A005843, A007395, A008586, A008592, A008602, A010692, A049094, A065607. Row n = 40 of A004247. Intersection of A008587 and A008590. After 0, subsequence of A005101. Sequence in context: A253692 A253699 A037975 * A039468 A181458 A069070 Adjacent sequences:  A317092 A317093 A317094 * A317096 A317097 A317098 KEYWORD nonn,easy AUTHOR Felix FrÃ¶hlich, Sep 07 2018 STATUS approved

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Last modified June 16 19:49 EDT 2021. Contains 345068 sequences. (Running on oeis4.)