[seqfan] Re: when is 1234...n a prime?

Frank Adams-Watters franktaw at netscape.net
Mon Oct 12 04:39:12 CEST 2015

Unless there are small examples, it is unlikely that even two are prime for the same n. The density for each is clearly < 1/n, and sum 1/n^2 is finite.

Franklin T. Adams-Watters

-----Original Message-----
From: Marc LeBrun <mlb at well.com>
To: Sequence Fanatics Discussion list <seqfan at list.seqfan.eu>
Sent: Sun, Oct 11, 2015 9:28 pm
Subject: [seqfan] Re: when is 1234...n a prime?

I'm idly curious, but lack the gumption to calculate:

What happens if you
append on the left?
1 21 321 4321 54321 ... 10987654321 1110987654321
121110987654321 ...

What happens if you reflect the numbers?

--> 211101987654321

121110987654321 --> 123456789011121

What's the least N for
which all four are


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