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 A087811 Numbers n such that ceiling(sqrt(n)) divides n. 15
 1, 2, 4, 6, 9, 12, 16, 20, 25, 30, 36, 42, 49, 56, 64, 72, 81, 90, 100, 110, 121, 132, 144, 156, 169, 182, 196, 210, 225, 240, 256, 272, 289, 306, 324, 342, 361, 380, 400, 420, 441, 462, 484, 506, 529, 552, 576, 600, 625, 650, 676, 702, 729, 756, 784, 812, 841 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Essentially the same as the quarter-squares A002620. Nonsquare elements of this sequence are given by A002378. - Max Alekseyev, Nov 27 2006 This also gives the number of ways to make change for "c" cents using only pennies, nickels and dimes. You must first set n=floor(c/5), to account for the 5-repetitive nature of the task. - Adam Sasson, Feb 09 2011 These are the segment boundaries of Oppermann's Conjecture (1882): n^2-n < p < n^2 < p < n^2+n. - Fred Daniel Kline, Apr 07 2011 A087811(n) is the number of (w,x,y) having all terms in {0..n} and w=2*x+y. - Clark Kimberling, Jun 04 2012 a(n+1) is also the number of points with integer coordinates inside a rectangle isosceles triangle with hypotenuse [0,n] (see A115065 for an equilateral triangle). - Michel Marcus, Aug 05 2013 a(n) = degree of generating polynomials of Galois numbers in (n+1)-dimensional vector space, defined as total number of subspaces in (n+1) space over GF(n) (see Mathematica procedure), when n is a power of a prime. - Artur Jasinski, Aug 31 2016, corrected by Robert Israel, Sep 23 2016 Also number of pairs (x,y) with 0 < x <= y <= n, x + y > n. - Ralf Steiner, Jan 05 2020 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..300 Wikipedia, Oppermann's conjecture Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA a(n) = (n + n mod 2)*(n + 2 - n mod 2)/4. Numbers of the form k^2 or k^2 - k. - Don Reble, Oct 17 2003 a(1) = 1, a(2) = 2, a(n) = n + a(n - 2). - Alonso del Arte, Jun 18 2005 G.f.: x/((1+x)*(1-x)^3). a(n) = (2*n*(n+2)-(-1)^n+1)/8. - Bruno Berselli, Feb 09 2011 G.f.: G(0)/(2*(1-x^2)*(1-x)), where G(k)= 1 + 1/(1 - x*(2*k+1)/(x*(2*k+2) + 1/G(k+1))); (continued fraction). - Sergei N. Gladkovskii, May 25 2013 a(n) = (C(n+2,2)-floor((n+2)/2))/2. - Mircea Merca, Nov 23 2013 a(n) = ((-1)^n*(-1 + (-1)^n*(1 + 2*n*(2 + n))))/8. - Fred Daniel Kline, Jan 06 2015 a(n) = Product_{k=1...n-1}(1 + 2 / (k + k mod 2)), n>=1. - Fred Daniel Kline, Oct 30 2016 E.g.f.: (1/4)*(x*(3 + x)*cosh(x) + (1 + 3*x + x^2)*sinh(x)). - Stefano Spezia, Jan 05 2020 MAPLE f:= gfun:-rectoproc({a(n)=n+a(n-2), a(1)=1, a(2)=2}, a(n), remember): map(f, [\$1..100]); # Robert Israel, Aug 31 2016 MATHEMATICA a := 1; a := 2; a[n_] := n + a[n - 2]; Table[a[n], {n, 57}] (* Alonso del Arte *) GaloisNumber[n_, q_] := Sum[QBinomial[n, m, q], {m, 0, n}]; aa = {}; Do[ sub = Table[GaloisNumber[m, n], {n, 0, 200}]; pp = InterpolatingPolynomial[sub, x]; pol = pp /. x -> n + 1; coef = CoefficientList[pol, n]; AppendTo[aa, Length[coef] - 1], {m, 2, 25}]; aa (* Artur Jasinski, Aug 31 2016 *) Select[Range, Divisible[#, Ceiling[Sqrt[#]]]&] (* or *) LinearRecurrence[ {2, 0, -2, 1}, {1, 2, 4, 6}, 60] (* Harvey P. Dale, Nov 06 2016 *) PROG (MAGMA) [ n: n in [1..841] | n mod Ceiling(Sqrt(n)) eq 0 ]; // Bruno Berselli, Feb 09 2011 (PARI) a(n)=(n+n%2)*(n+2-n%2)/4 \\ Charles R Greathouse IV, Apr 03 2012 (PARI) j=0; for(k=1, 850, s=sqrtint(4*k+1); if(s>j, j=s; print1(k, ", "))) \\ Hugo Pfoertner, Sep 17 2018 (Haskell) a087811 n = (n + n `mod` 2) * (n + 2 - n `mod` 2) `div` 4 -- Reinhard Zumkeller, Oct 27 2012 CROSSREFS Cf. A002378, A002620, A003059, A110835, A316841. Subsequence of A006446. Sequence in context: A083392 A076921 A002620 * A025699 A224813 A224812 Adjacent sequences:  A087808 A087809 A087810 * A087812 A087813 A087814 KEYWORD nonn,easy AUTHOR Reinhard Zumkeller, Oct 16 2003 STATUS approved

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Last modified June 16 04:44 EDT 2021. Contains 345056 sequences. (Running on oeis4.)