Ask HN: Why do parking tickets across lots of cities follow a similar # pattern?

[davidhegarty]

We noticed something interesting as we were looking at parking ticket numbers.

The sequence of parking ticket numbers follows a similar pattern across lots of different cities.

We thought it was because cities shared a similar software vendor... but it looks to be more extensive than that.

Anyone have any ideas why a large number of cities have the following parking ticket sequence:

827778501 ([phone] ([phone] ([phone] ([phone] ([phone] ([phone] (+4)

...and then continues to add '11' six times, and then adds a '4', and keeps repeating.

Is there a reason for this? Is it a common sequence in other places too?

[madcaptenor]

The last digit may be a check digit. In particular, it might be the remainder upon dividing the number that the other digits make by 7. For example 82777850 divided by 7 leaves a remainder of 1. Some examples of this check digit scheme:

http://nrich.maths.org/2036 http://www.cbp.gov/sites/default/files/documents/in_bond_che... (with a clunky implementation) http://www.con-way.com/en/tools_pricing/freight/fr_tools/tra...

[davidhegarty]

BINGO!!! You da man. That's seems to work for all my examples.

Thanks!