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A195265 Trajectory of 20 under iteration of the map x -> A080670(x). 3
20, 225, 3252, 223271, 297699, 399233, 715623, 3263907, 32347303, 160720129, 1153139393, 72171972859, 736728093411, 3245576031137, 11295052366467, 310807934835791, 1789205424940407, 31745337977379983, 1122916740775279751, 7251536377635958081, 151243563319717018007 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The table that I submitted for A195264 (see the second link here) is still actively maintained by me. That includes all unknown-outcome evolutions starting with numbers up to 10000. If you click in that table on the 'unknown' beside the number 20 it will give you the current state of the evolution of the number 20. There was a bottleneck at Alonso20(102) [= A195265(103); we're using offset zero for our evolutions] involving a 62-digit factor, cracked by a Mathew in MersenneForum on August 13. Sean A. Irvine subsequently extended that to Alonso20(109). The unfactored composite in Alonso20(110) is 178 digits long. I maintain links to sorted lists of unfactored composites at the bottom of the table. If anyone can factor any of these composites, submit the factorization to factordb.com and I will (eventually) find it; a personal heads-up would of course be appreciated. - Hans Havermann, Oct 27 2013

LINKS

Hans Havermann, Table of n, a(n) for n = 1..110

J. H. Conway, Five $1000 Problems (This sequence is mentioned in the fifth problem)

Hans Havermann, Table of n, A195264(n) for n = 1..10000 (includes links to lengthy (>40) and unknown-outcome evolutions, and a list of unfactored composites in the unknowns' last step)

EXAMPLE

20 = 2^2*5 -> 225 = 3^2*5^2 -> 3252 = 2^2*3*271 -> 223271 ...

MAPLE

# See A195266

MATHEMATICA

A080670[n_] := ToExpression@StringJoin[ToString/@Flatten[DeleteCases[FactorInteger[n], 1, -1]]]; NestWhileList[A080670, i = 1; 20, (PrintTemporary[{i++, #}]; ! PrimeQ[#]) &, 1, 40] (* Wouter Meeussen, Oct 27 2013 *)

CROSSREFS

Cf. A037274, A080670, A195264, A195266, A230305.

Sequence in context: A054329 A112503 A007160 * A264876 A023018 A073386

Adjacent sequences:  A195262 A195263 A195264 * A195266 A195267 A195268

KEYWORD

nonn,base

AUTHOR

N. J. A. Sloane, Sep 14 2011, based on a posting to the Sequence Fans Mailing List by Alonso del Arte

EXTENSIONS

Alonso del Arte computed 40 terms, D. S. McNeil extended it to 66 terms, Sean A. Irvine to 70 terms, Hans Havermann (Oct 27 2013) to 110 terms.

STATUS

approved

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Last modified April 25 15:43 EDT 2017. Contains 285416 sequences.