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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
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 (list; graph; refs; listen; history; text; internal format)
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
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
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

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Last modified May 19 09:42 EDT 2024. Contains 372683 sequences. (Running on oeis4.)