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A130519
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a(n) = Sum_{k=0..n} floor(k/4). (Partial sums of A002265.)
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16
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0, 0, 0, 0, 1, 2, 3, 4, 6, 8, 10, 12, 15, 18, 21, 24, 28, 32, 36, 40, 45, 50, 55, 60, 66, 72, 78, 84, 91, 98, 105, 112, 120, 128, 136, 144, 153, 162, 171, 180, 190, 200, 210, 220, 231, 242, 253, 264, 276, 288, 300, 312, 325, 338, 351, 364, 378, 392, 406, 420, 435, 450
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OFFSET
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0,6
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COMMENTS
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Complementary to A130482 with respect to triangular numbers, in that A130482(n) + 4*a(n) = n(n+1)/2 = A000217(n).
Disregarding the first three 0's the resulting sequence a'(n) is the sum of the positive integers <= n that have the same residue modulo 4 as n. This is the additive counterpart of the quadruple factorial numbers. - Peter Luschny, Jul 06 2011
Column sums of (shift of rows = 4):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 ...
1 2 3 4 5 6 7 8 9 10 ...
1 2 3 4 5 6 ...
1 2 ...
.......................................
---------------------------------------
1 2 3 4 6 8 10 12 15 18 21 24 28 32 ...
shift of rows = 1 see A000217
shift of rows = 2 see A002620
shift of rows = 3 see A001840
shift of rows = 5 see A130520 - Heinrich Ludwig, Dec 23 2017
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..10000
Bakir Farhi, On the Representation of the Natural Numbers as the Sum of Three Terms of the Sequence floor(n^2/a), Journal of Integer Sequences, Vol. 16 (2013), #13.6.4.
Index entries for linear recurrences with constant coefficients, signature (2,-1,0,1,-2,1).
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FORMULA
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G.f.: x^4/((1-x^4)*(1-x)^2) = x^4/((1+x)*(1+x^2)*(1-x)^3).
a(n) = +2*a(n-1) -1*a(n-2) +1*a(n-4) -2*a(n-5) +1*a(n-6).
a(n) = floor(n/4)*(n - 1 - 2*floor(n/4)) = A002265(n)*(n - 1 - 2*A002265(n)).
a(n) = (1/2)*A002265(n)*(n - 2 + A010873(n)).
a(n) = floor((n-1)^2/8). - Mitch Harris, Sep 08 2008
a(n) = round(n*(n-2)/8) = round((n^2-2*n-1)/8) = ceiling((n+1)*(n-3)/8). - Mircea Merca, Nov 28 2010
a(n) = A001972(n-4), n>3. - Franklin T. Adams-Watters, Jul 10 2009
a(n) = a(n-4)+n-3, n>3. - Mircea Merca, Nov 28 2010
Euler transform of length 4 sequence [ 2, 0, 0, 1]. - Michael Somos, Oct 14 2011
a(n) = a(2-n) for all n in Z. - Michael Somos, Oct 14 2011
a(n) = A214734(n, 1, 4). - Renzo Benedetti, Aug 27 2012
a(4n) = A000384(n), a(4n+1) = A001105(n), a(4n+2) = A014105(n), a(4n+3) = A046092(n). - Philippe Deléham, Mar 26 2013
a(n) = Sum_{i=1..ceiling(n/2)-1} (i mod 2) * (n - 2*i - 1). - Wesley Ivan Hurt, Jan 23 2014
a(n) = ( 2*n^2-4*n-1+(-1)^n+2*((-1)^((2*n-1+(-1)^n)/4)-(-1)^((6*n-1+(-1)^n)/4)) )/16 = ( 2*n*(n-2) - (1-(-1)^n)*(1-2*i^(n*(n-1))) )/16, where i=sqrt(-1). - Luce ETIENNE, Aug 29 2014
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EXAMPLE
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x^4 + 2*x^5 + 3*x^6 + 4*x^7 + 6*x^8 + 8*x^9 + 10*x^10 + 12*x^11 + ...
[ n] a(n)
---------
[ 4] 1
[ 5] 2
[ 6] 3
[ 7] 4
[ 8] 1 + 5
[ 9] 2 + 6
[10] 3 + 7
[11] 4 + 8
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MAPLE
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quadsum := n -> add(k, k = select(k -> k mod 4 = n mod 4, [$1 .. n])):
A130519 := n ->`if`(n<3, 0, quadsum(n-3)); seq(A130519(n), n=0..58); # Peter Luschny, Jul 06 2011
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MATHEMATICA
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a[ n_] := Quotient[ (n - 1)^2, 8]; (* Michael Somos, Oct 14 2011 *)
LinearRecurrence[{2, -1, 0, 1, -2, 1}, {0, 0, 0, 0, 1, 2}, 80] (* Vladimir Joseph Stephan Orlovsky, Feb 24 2012 *)
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PROG
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(PARI) {a(n) = (n - 1)^2 \ 8}; /* Michael Somos, Oct 14 2011 */
(MAGMA) [Round(n*(n-2)/8): n in [0..70]]; // Vincenzo Librandi, Jun 25 2011
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CROSSREFS
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Cf. A000217, A001840, A002264, A002265, A002266, A002620, A004526, A010872, A010873, A010874, A130481, A130483, A130520.
Sequence in context: A056168 A054041 A019293 * A001972 A005705 A139542
Adjacent sequences: A130516 A130517 A130518 * A130520 A130521 A130522
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KEYWORD
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nonn,easy,changed
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AUTHOR
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Hieronymus Fischer, Jun 01 2007
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EXTENSIONS
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Partially edited by R. J. Mathar, Jul 11 2009
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STATUS
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approved
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