A110409 Number of times repeated reverse concatenation of n followed by n gives a prime, where n == 1,3,7 or 9 (mod 10), or 0 if no such prime exists.

1, 0, 0, 0, 0, 10, 3, 7, 0, 3, 0, 10, 3, 0, 22, 0, 51, 7, 9, 10, 0, 412, 0, 16, 18, 0, 3, 0, 3, 3, 0, 9, 0, 3, 0, 3, 4, 0, 3, 0, 0

Offset

0,6

Comments

Except for the first term every nonzero term is >1.

The larger numbers are probable primes. - Joshua Zucker, May 10 2006

The sequence probably continues 0 0 10 0 0 0 0 0 0 4 0 0 0 130 6 0 0 0 4 0 0 0 0 0 6 6 0 0 4 0 4 0 0 0 0 0 10 10 34 0 0 0 0 but the 0's in that list that correspond to 103, 107, 113, 119, 133, 143, 157, 169, 187, 203, 209, are not proved (but if there is a term there, it is more than 500). - Joshua Zucker, May 10 2006

Not only must each nonzero term be >1 (to avoid divisibility by 11), it also cannot equal 2 (mod 3) to avoid divisibility by 3. - Joshua Zucker, May 10 2006

Links

Table of n, a(n) for n=0..40.

Example

The term corresponding to 19 is 7, as 7 concatenation of 91 followed by 19 is the least such prime. (9191919191919119 is a prime).

Crossrefs

Cf. A110408.

Sequence in context: A338288 A330008 A335844 * A361377 A309382 A064211

Adjacent sequences: A110406 A110407 A110408 * A110410 A110411 A110412

Keyword

base,more,nonn

Author

Amarnath Murthy, Jul 30 2005

Extensions

More terms from Joshua Zucker, May 10 2006

Edited by T. D. Noe, Oct 30 2008

Status

approved