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A075022 a(1) = 1; for n>1, a(n) = the largest prime divisor of the number C(n) formed from the concatenation of 1,2,3,... up to n. 8
1, 3, 41, 617, 823, 643, 9721, 14593, 3803, 1234567891, 630803, 2110805449, 869211457, 205761315168520219, 8230452606740808761, 1231026625769, 584538396786764503, 801309546900123763, 833929457045867563 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Zak Seidov, Table of n, a(n) for n = 1..50.

EXAMPLE

a(4) = 617 since 1234 = 2*617.

MATHEMATICA

a = {}; b = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[k]]], {k, 1, Length[w]}]; p = FromDigits[a]; AppendTo[b, First[Last[FactorInteger[p]]]], {n, 1, 25}]; b - Artur Jasinski, Apr 04 2008

Table[FactorInteger[FromDigits[Flatten[IntegerDigits/@Range[n]]]][[-1, 1]], {n, 20}] (* Harvey P. Dale, Aug 31 2015 *)

CROSSREFS

Cf. A075019, A075020, A075021.

Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138958, A138959, A138960, A138961, A138962.

Sequence in context: A155672 A073844 A138961 * A300281 A012175 A007313

Adjacent sequences:  A075019 A075020 A075021 * A075023 A075024 A075025

KEYWORD

base,nonn

AUTHOR

Amarnath Murthy, Sep 01 2002

EXTENSIONS

More terms from Sascha Kurz, Jan 03 2003

STATUS

approved

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Last modified July 20 16:24 EDT 2019. Contains 325185 sequences. (Running on oeis4.)