its simple ur logic was correct but i think u didnt think in this way
L2 = { X1/0 , X1/1,X1/2,X1/3,X1/4.............}
the means L2= {infinity, X1,X0.5,X0.75,X0.25...................} here these are not in AP (unequal common difference),so this might be the condition for NON-REGULARITY
if i went wrong let me know