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 A108416 Triangle read by rows: T(n,k) counts the k-subsets of the n-th roots of 1 with absolute value of sum=1. 1

%I

%S 0,0,1,0,2,0,3,0,4,0,0,5,0,0,6,6,12,0,7,0,0,0,8,0,24,0,0,9,9,0,18,0,

%T 10,0,40,10,60,0,11,0,0,0,0,0,12,12,60,72,144,120,0,13,0,0,0,0,0,0,14,

%U 0,84,0,210,14,280,0,15,15,0,75,60,30,105,0,16,0,112,0,336,0,560,0,0,17,0,0

%N Triangle read by rows: T(n,k) counts the k-subsets of the n-th roots of 1 with absolute value of sum=1.

%C Row n is divisible by n (rotation symmetry).

%C Row sums: A108417.

%e T(6,2)=6, counting {1,3}, {1,5}, {2,4}, {2,6}, {3,5}, {4,6}.

%e Table starts:

%e 0,

%e 0, 1,

%e 0, 2, 0,

%e 0, 3, 3, 0,

%e 0, 4, 0, 4, 0,

%e 0, 5, 0, 0, 5, 0,

%e 0, 6, 6,12, 6, 6, 0,

%e 0, 7, 0, 0, 0, 0, 7, 0,

%e 0, 8, 0,24, 0,24, 0, 8, 0,

%e 0, 9, 9, 0,18,18, 0, 9, 9, 0

%t <<DiscreteMath`Combinatorica`; Table[Count[KSubsets[Range[n], k], q_List/;Chop[ -1+Abs[Plus @@ (E^((2.*Pi*I*q)/n))]] === 0], {n, 16}, {k, 0, n}]

%Y Cf. A107754, A103314, A108417.

%K nonn,tabl

%O 0,5

%A _Wouter Meeussen_, Jun 02 2005

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Last modified February 6 16:56 EST 2023. Contains 360110 sequences. (Running on oeis4.)