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FORMULA
| Let X = the 7x7 incidence matrix of the Fano plane PG(2,2): [1,1,1,0,0,0,0; 1,0,0,1,1,0,0; 1,0,0,0,0,1,1; 0,1,0,1,0,1,0; 0,1,0,0,1,0,1; 0,0,1,1,0,0,1; 0,0,1,0,1,1,0]. a(n) = the upper left term in X^n.
Empirical a(n)=3*a(n-1)+2*a(n-2)-6*a(n-3). G.f.: x*(1-6*x^2)/(1-3*x-2*x^2+6*x^3). [Colin Barker, Jan 26 2012]
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