Right you are! Thanks for catching that. Jim On Sun, Sep 22, 2019 at 10:47 PM Michael Kleber <michael.kleber@gmail.com> wrote:
Jim, I think you slipped into thinking about 7-digit zip codes for a bit there. It's only the initial *two* digits, not three, that can be chosen freely; the third digit is the one constrained to be its own rotation.
So the number of "evil twins" is (7^5 - 7*7*5)/2 = 8281, which leaves 91719 available under this rule.
--Michael
On Sun, Sep 22, 2019 at 12:22 PM James Propp <jamespropp@gmail.com> wrote:
Neat fact! (For those unfamiliar with regex notation, "[0125689]{5}" matches every 5-digit string containing no 3's, 4's, or 7's.)
The number of zip codes compatible with this rule can be computed as follows. There are 10^5 - 7^5 = 83,193 zip codes that contain at least one 3, 4, or 7, and hence pose no issues. Of the remaining 7^5 = 16,807 zip codes containing no 3, 4, or 7, there are 5 * 7^3 = 1715 that are carried to themselves under 180 degree rotation, because the middle digit must be 0, 1, 2, 5, or 8, the first three digits can be arbitrary elements of {0,1,2,5,6,8,9}, and the last three digits are determined by the first three digits. That means that there are 16,807 - 1715 = 15,092 zip codes that (like the number 15092 itself!) have an "evil twin", and half of these (15,092 / 2 = 7546) must go unused. So the total number of possible zip codes would be 10^5 - 7546 = 92,454 if this were the only governing constraint.
Is this calculation correct?
Perhaps a more interesting question is, how does the postal service decide which of the twins is good and which is evil?
Also, are there any other mathematically interesting constraints on zip codes?
By the way, according to
https://www.traveltrivia.com/answer-how-many-zip-codes-are-there-in-the-u-s/
, the number of zip codes currently in use is around 42,000.
Jim Propp
On Sun, Sep 22, 2019 at 11:19 AM Michael Kleber < michael.kleber@gmail.com> wrote:
Wow! Thank you, Leo, this is an outstanding bit of zip code trivia (which is new to me).
--Michael
On Sun, Sep 22, 2019 at 3:19 AM Leo Broukhis <leob@mailcom.com> wrote:
Out of two strings matching [0125689]{5}, where the second string corresponds to the upside-down reading of the first, no more than one string will be a valid zip code.
Leo
On Fri, Sep 13, 2019 at 12:19 PM Thane Plambeck <tplambeck@gmail.com
wrote:
The G4Ger who did the zip code act is named David Rosdeitcher. He lives near Boulder, CO.
He has several flat Earth videos on YouTube.
On Fri, Sep 13, 2019 at 11:52 AM Andres Valloud < avalloud@smalltalk.comcastbiz.net> wrote:
Regarding the subject, it appears US (basic) zip codes are effectively character digit strings of length 5 (that is, [0-9]{5}). Some state there are 42k active zip codes, presumably of the five digit variety.
On 9/13/19 6:28 , Bill Gosper wrote: > G4G once had an entertainer named "Mr. Zipcode"(?). > You tell him yours and he tells you where, and then goes on to describe > places to eat, etc. (He struggled with Ray Solomonoff's obscure town in > upstate New York. Or maybe some offstage confederate struggled with > https://m.usps.com/m/ZipLookupAction?search=zip .) > > Impressive, but maybe not 10^-5 impressive. > I just mistakenly tried 44007, which seems to be Nantes, > the site of a French Revolution atrocity. Does Mr. Zipcode mention > atrocities? —rwg > _______________________________________________ > math-fun mailing list > math-fun@mailman.xmission.com > https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun > _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun
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