a(n) = 5*(10^(2*n+1) - 1)/9 - 5*10^n.A332150a(n) = 5*(10^(2*n+1) - 1)/9 - 5*10^n.Terms a(0) =0a(1) =505a(2) =55055a(3) =5550555a(4) =555505555a(5) =55555055555External referencesoeis: A332150