If a number conforms to a certain rule, I call it a Well-toned Number™. Use the following examples to find the rule:
Here is a CSV version:
Well-toned Numbers™,Not Well-toned Numbers™ 36,49 343,729 2000,1000 13597,13579 254708,125678 807452,876521 5551205,5552106 8675190,8675309 138076942564,138076942546 942564138076,942546138076
The puzzle relies on the series' inbuilt assumption, that each number can be tested for whether it is a Well-toned Number™ on its own. In particular, a number's relationship to other numbers in the sequence is irrelevant.
These are not the only examples of Well-toned Numbers™ (or not Well-toned Numbers™), more can be found.
Maybe @DarfNader is onto something...