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 A217571 a(n) = (2*n*(n+5) + (2*n+1)*(-1)^n - 1)/8. 3
 1, 4, 5, 10, 11, 18, 19, 28, 29, 40, 41, 54, 55, 70, 71, 88, 89, 108, 109, 130, 131, 154, 155, 180, 181, 208, 209, 238, 239, 270, 271, 304, 305, 340, 341, 378, 379, 418, 419, 460, 461, 504, 505, 550, 551, 598, 599, 648, 649, 700, 701, 754, 755, 810, 811, 868 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS One of four sequences given by classifying natural numbers according to the value of floor(sqrt(n)). See Sato link and sequences A005563, A217570, A217575. Numbers n such that floor(sqrt(n)) = floor(n/floor(sqrt(n))) = floor(n/(floor(sqrt(n)) + 2)) + 1. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Takumi Sato, Classification of Natural Numbers [Wayback Machine link] Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1). FORMULA G.f.: x*(1+3*x-x^2-x^3)/((1+x)^2*(1-x)^3). - Bruno Berselli, Oct 11 2012 From Stefano Spezia, Dec 14 2019: (Start) E.g.f.: (x*(5+x)*cosh(x) - (1-7*x-x^2)*sinh(x))/4. a(n) = a(n-1) + 1 for n odd. a(n) = a(n-1) + n + 1 for n even. a(2*n) = A028552(n). a(2*n+1) = A028387(n). (End) EXAMPLE From Stefano Spezia, Dec 14 2019: (Start) Illustration of the initial terms: o      o        o        o           o      o o o    o o o    o o o       o o o                 o        o           o                      o o o o o   o o o o o                                      o (1)   (4)      (5)     (10)        (11) (End) MAPLE seq( (2*n^2 +10*n -1 +(-1)^n*(2*n+1))/8, n=1..60); # G. C. Greubel, Dec 19 2019 MATHEMATICA CoefficientList[Series[(1 + 3*x - x^2 - x^3)/((1 + x)^2*(1 - x)^3), {x, 0, 40}], x] (* Vincenzo Librandi, Dec 15 2012 *) a[1]=1; a[n_]:=If[EvenQ[n], a[n-1]+1+n, a[n-1]+1]; Array[a, 56] (* Stefano Spezia, Dec 18 2019 *) PROG (Visual Basic in Excel) Sub A217571() Dim x As Long, n As Long, y As Long, i As Long x = InputBox("Count to") For n = 1 To x y = Int(Sqr(n)) If y = Int(n / y) Then GoTo L1 GoTo L2 L1: If y = Int(n / (y + 2)) + 1 Then i = i + 1 Cells(i, 1) = n End If L2: Next n End Sub (MAGMA) [n: n in [1..900] | Floor(n/Isqrt(n)) eq Floor(n/(Isqrt(n)+2))+1]; // Bruno Berselli, Oct 10 2012 (Maxima) makelist((2*n*(n+5)+(2*n+1)*(-1)^n-1)/8, n, 1, 56); /* Martin Ettl, Oct 15 2012 */ (MAGMA) I:=[1, 4, 5, 10, 11]; [n le 5 select I[n] else Self(n-1) + 2*Self(n-2) - 2*Self(n-3) - Self(n-4) + Self(n-5): n in [1..60]]; // Vincenzo Librandi, Dec 15 2012 (PARI) vector(60, n, (2*n^2 +10*n -1 +(-1)^n*(2*n+1))/8 ) \\ G. C. Greubel, Dec 19 2019 (Sage) [(2*n^2 +10*n -1 +(-1)^n*(2*n+1))/8 for n in (1..60)] # G. C. Greubel, Dec 19 2019 (GAP) List([1..60], n-> (2*n^2 +10*n -1 +(-1)^n*(2*n+1))/8 ); # G. C. Greubel, Dec 19 2019 CROSSREFS Cf. A005563, A028387, A028552, A217570, A217575. Sequence in context: A327311 A151735 A050039 * A058335 A222353 A094415 Adjacent sequences:  A217568 A217569 A217570 * A217572 A217573 A217574 KEYWORD nonn,easy AUTHOR Takumi Sato, Oct 07 2012 EXTENSIONS Definition by Bruno Berselli, Oct 11 2012 STATUS approved

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Last modified January 21 10:25 EST 2022. Contains 350477 sequences. (Running on oeis4.)