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A075019
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a(1) = 1; for n>1, a(n) = the smallest prime divisor of the number C(n) formed from the concatenation of 1,2,3,... up to n.
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9
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1, 2, 3, 2, 3, 2, 127, 2, 3, 2, 3, 2, 113, 2, 3, 2, 3, 2, 13, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 29, 2, 3, 2, 3, 2, 71, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 23, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 10386763, 2, 3, 2, 3, 2, 397, 2, 3, 2, 3, 2, 37907, 2, 3, 2, 3, 2, 73, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 37, 2, 3, 2
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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EXAMPLE
| a(5)= 5, 5 is the smallest prime divisor of 12345.
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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[First[FactorInteger[p]]]], {n, 1, 25}]; b - Artur Jasinski (grafix(AT)csl.pl), Apr 04 2008
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CROSSREFS
| Cf. A075020.
Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138958, A138959, A138960, A138961, A138962.
Sequence in context: A108656 A164962 A119880 * A138960 A115397 A200323
Adjacent sequences: A075016 A075017 A075018 * A075020 A075021 A075022
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KEYWORD
| base,nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Sep 01 2002
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EXTENSIONS
| More terms from Sascha Kurz (sascha.kurz(AT)uni-bayreuth.de), Jan 03 2003
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