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 A276451 Number of 2-orbits of the cyclic group C_4 for a bi-colored square n X n grid with n squares of one color. 4
 0, 1, 2, 12, 30, 408, 1012, 17920, 45600, 1059380, 2730756, 78115884, 203235032, 6917206576, 18113945256, 714851008512, 1881039165696, 84449819514060, 223049005408900, 11225502116862880, 29736777118603962, 1658138369930988088, 4403069737450280832 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS For a definition and examples of this problem see the comment section of A276449. The present sequence a(n) gives the number of 2-orbits of such 2-color boards with n squares of one color under C_4. LINKS Hong-Chang Wang, Table of n, a(n) for n = 1..100 Hong-Chang Wang, Example for n = 4 FORMULA a(n) = (binomial(2*i*i,i) - A276449(n))/2, for n = 2*i. a(n) = (binomial(2*i*(i+1),i) - A276449(n))/2, for n = 2*i+1. EXAMPLE n = 4: one of the two 2-orbits is (o white, + black) + o + o o o o + o o o o + o o o o o o o o o o + o + o + + o o o, and one can take the first one as a representative. For n = 3 there are a(3) = 2 2-orbits, represented by + o o o o o o + o and + + + o o + o o o. The orbit structure for n=3 is 1^0 2^2 4^20; see A276449(3) = 0, a(3) = 2, A276452(3) = 20. For the 12 2-orbits for n=4, see the representatives given in the link. MATHEMATICA Table[(Function[j, Binomial[2 j (j + Boole@ OddQ@ n), j]]@ Floor[n/2] - If[MemberQ[{2, 3}, #], 0, Function[i, Binomial[(2 i) (2 i + #), i]]@ Floor[n/4]] &@ Mod[n, 4])/2, {n, 23}] (* Michael De Vlieger, Sep 07 2016 *) PROG (Python) import math def nCr(n, r): f = math.factorial return f(n) / f(r) / f(n-r) # main program for j in range(101): i = j/2 if j%2==0: b = nCr(2*i*i, i) else: b = nCr(2*i*(i+1), i) if j%4==0: c = nCr((j*j/4), (j/4)) elif j%4==1: c = nCr(((j-1)/2)*((j-1)/2+1), ((j-1)/4)) else: c = 0 print(str(j)+" "+str((b-c)/2)) CROSSREFS Cf. A014062, A276449, A276452, A276454. Sequence in context: A019258 A124903 A197954 * A075352 A116655 A361760 Adjacent sequences: A276448 A276449 A276450 * A276452 A276453 A276454 KEYWORD nonn,easy AUTHOR Chiang, Tung-Ying, Jason Y.S. Chiu, Hong-Chang Wang, Jiangshan Sun, Sep 03 2016 EXTENSIONS Edited: Wolfdieter Lang, Oct 02 2016 STATUS approved

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