Trajectory of 21 under map x->x + (x-with-digits-reversed).

Terms

a(0) =21a(1) =33a(2) =66a(3) =132a(4) =363a(5) =726a(6) =1353a(7) =4884a(8) =9768a(9) =18447a(10) =92928a(11) =175857a(12) =934428a(13) =1758867a(14) =9447438a(15) =17794887a(16) =96644658a(17) =182289327a(18) =906271608a(19) =1712444217a(20) =8836886388a(21) =17673772776a(22) =85401510447a(23) =159803020905