The On-Line Encyclopedia of Integer Sequences (OEIS)

Revision History for A326344

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A326344

a(1) = 1. Thereafter, if n is prime, a(n) is the next prime after a(n-1), but written backwards. If n is not prime, a(n) is the next composite after a(n-1), written backwards.
(history; published version)

#83 by Harvey P. Dale at Sun Apr 05 13:26:25 EDT 2020

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#82 by Harvey P. Dale at Sun Apr 05 13:26:17 EDT 2020

MATHEMATICA

ncp[{n_, a_}]:=Module[{k=1}, {n+1, If[PrimeQ[n+1], IntegerReverse[NextPrime[ a]], While[!CompositeQ[k+a], k++]; IntegerReverse[k+a]]}]; NestList[ncp, {1, 1}, 80][[All, 2]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Apr 05 2020 *)

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#81 by N. J. A. Sloane at Fri Sep 27 22:33:48 EDT 2019

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#80 by N. J. A. Sloane at Fri Sep 27 22:33:44 EDT 2019

CROSSREFS

For analogs in bases 3,5,6,7,10 see A326894, A327464, A327463, A327241, A326344 = the present sequence.

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#79 by N. J. A. Sloane at Fri Sep 27 22:04:56 EDT 2019

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#78 by N. J. A. Sloane at Fri Sep 27 22:04:54 EDT 2019

CROSSREFS

For analogs in bases 3 , 6, and 7 see A326894 , A327463, and A327241.

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#77 by N. J. A. Sloane at Fri Sep 27 21:00:00 EDT 2019

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#76 by N. J. A. Sloane at Fri Sep 27 20:59:58 EDT 2019

CROSSREFS

For analogs in bases 3 and 7 see A326894 and A327241.

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#75 by N. J. A. Sloane at Fri Sep 13 13:57:02 EDT 2019

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#74 by N. J. A. Sloane at Fri Sep 13 13:56:59 EDT 2019

COMMENTS

The first two-digit a(n) occurs at n = 17. The first three-digit a(n) occurs at n = 643. According to Michel Marcus, in the first 10^8 terms, a(n) never exceeds 909. It is unknown whether it can or will exceed 909. See comment by _Rémy Sigrist_ belownow known that this is the maximal value (see the Weimholt link).

The Terms a(n) > 800 occur at indices (649, 3132, [3595], 3596, [6805], 6806, 7344, 8233, [8234], [11173], 11174, 12619, 13687, 14089, ...). (Subsequent indices are > 20000. Indices in [.] correspond to a non-maximal value, i.e., a(n+-1) > a(n).) The corresponding values are in the set {804, 806, 807, 808, 809, 904, 907} and occur as part of one of the following subsequences: (maxima starred)

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