222121domain: NAppears in sequencesNumbers for which the sum of reciprocals of square of digits is an integer.at n=25A1824523 x 3 magic square containing seven squares, read by rows.at n=8A221669Shortest (x+1,2x)-code of n.at n=18A233135List of co-Lyndon words on {1,2} sorted first by length and then lexicographically.at n=20A329318