a(n) = Sum_{d|n} (d^2-1)^n.A367552a(n) = Sum_{d|n} (d^2-1)^n.Terms a(0) =0a(1) =9a(2) =512a(3) =50706a(4) =7962624a(5) =1838528498a(6) =587068342272External referencesoeis: A367552