a(n) = 7*(10^(2*n) - 1)/33.A153848a(n) = 7*(10^(2*n) - 1)/33.Terms a(0) =21a(1) =2121a(2) =212121a(3) =21212121a(4) =2121212121a(5) =212121212121External referencesoeis: A153848