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A212892
a(n) = n^4/8 if n is even, a(n) = (n^2-1)^2/8 if n is odd.
3
0, 0, 2, 8, 32, 72, 162, 288, 512, 800, 1250, 1800, 2592, 3528, 4802, 6272, 8192, 10368, 13122, 16200, 20000, 24200, 29282, 34848, 41472, 48672, 57122, 66248, 76832, 88200, 101250, 115200, 131072, 147968, 167042, 187272, 209952, 233928, 260642, 288800, 320000, 352800, 388962, 426888
OFFSET
0,3
COMMENTS
The sequence a(0)=0, a(1)=2, a(2)=8, a(3)=32, ... arises as the number of quadruples (w,x,y,z) with all terms in {0,...,n} and w-x, x-y, and y-z all odd. For a guide to related sequences, see A211795.
The sequence a(3)=2, a(4)=8, a(5)=32, ... is the number of L3 configurations in an n X n permutation array. An L3 configuration is defined to be a set of 3 equally-spaced, collinear points in a permutation array. L3 configurations were first enumerated by Davies in his study of the density of Costas arrays. They constitute a violation of the definition of a Costas array, so Costas arrays cannot have any. Davies's work went unpublished until it appeared in the survey paper by Drakakis. - Bill Correll, Jr., Jun 07 2014
Every term is even.
LINKS
K. Drakakis, A review of Costas arrays, Journal of Applied Mathematics, pp. 1-32, 2006, Article ID 26385.
FORMULA
If the sequence begins a(0)=0, a(1)=2, a(2)=8, a(3)=32, ...: a(n) = 2*a(n-1)+2*a(n-2)-6*a(n-3)+6*a(n-5)-2*a(n-6)-2*a(n-7)+a(n-8). G.f.: f(x)/g(x), where f(x) = x+2*x^2+6*x^3+2*x^4+x^5 and g(x) = ((1-x)^5)*(1+x)^3.
Sum_{n>=2} 1/a(n) = Pi^4/180 + Pi^2/6 - 3/2. - Amiram Eldar, Sep 08 2022
a(n) = 2*floor(n^2/4)^2 = 2*A030179(n). - Ridouane Oudra, Sep 12 2023
MAPLE
f:=n->if n mod 2 = 0 then n^4/8 else (n^2-1)^2/8; fi; [seq(f(n), n=0..50)]; # N. J. A. Sloane, Mar 06 2015
MATHEMATICA
(* If the sequence begins a(0)=0, a(1)=2, a(2)=8, a(3)=32, ... *)
t = Compile[{{n, _Integer}}, Module[{s = 0}, (Do[If[Mod[w - x, 2] == Mod[x - y, 2] == Mod[y - z, 2] == 1, s = s + 1], {w, 0, n}, {x, 0, n}, {y, 0, n}, {z, 0, n}]; s)]];
m = Map[t[#] &, Range[0, 40]] (* A212892 *)
m/2 (* integers *)
CROSSREFS
Cf. A211795.
Cf. A030179.
Sequence in context: A331823 A006552 A134753 * A302936 A227971 A267661
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, May 30 2012, and Bill Correll, Jr., Jun 07 2014
EXTENSIONS
Formed by merging two entries that arose in different contexts. Thanks to Alois P. Heinz, Mar 04 2015 for noticing that the sequences were essentially identical. - N. J. A. Sloane, Mar 06 2015
STATUS
approved