Search: id:A086243
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%I A086243
%S A086243 2,14,68,604,4312
%N A086243 Number of invertible (-1,0,1) n X n matrices having (Tij=-Tji; i<j) such 
               that all T^k (k= 1..12) are also (-1,0,1) invertible matrices having 
               (Tij=-Tji; i<j).
%C A086243 A set of matrices closed under exponentiation.
%C A086243 The powers T^k are themselves all members of the set that is counted.
%t A086243 (* trimatsig[ ] : see A072110 *) n=3; it=triamatsig/@(-1+IntegerDigits[Range[0, 
               -1+3^(n(n+1)/2)], 3, n(n+1)/2]); result[n]=Cases[it, (q_?MatrixQ)/
               ; Det[q]=!=0 && And@@Table[Union[Flatten[{(temp=MatrixPower[q, k]), 
               {-1, 0, 1}}]]==={-1, 0, 1} && ((1-IdentityMatrix[n])temp===-Transpose[(1-IdentityMatrix[n])temp]), 
               {k, 12}]]; Length[%]
%Y A086243 Cf. A072110.
%Y A086243 Sequence in context: A109869 A084132 A084770 this_sequence A072888 A094583 
               A002058
%Y A086243 Adjacent sequences: A086240 A086241 A086242 this_sequence A086244 A086245 
               A086246
%K A086243 hard,nonn
%O A086243 1,1
%A A086243 Wouter Meeussen (wouter.meeussen(AT)pandora.be), Aug 28 2003

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