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Search: id:A109837
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| A109837 |
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Smallest prime factor of the reverse concatenation of the first n odd numbers. |
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+0
4
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| 31, 3, 17, 7, 3, 1579, 17, 3, 2677, 131, 3, 54049423, 47, 3, 487, 25541, 3, 7, 211, 3, 31, 622639, 3, 498508631, 7, 3, 1259, 193, 3, 7669, 16229, 3, 13, 887377, 3, 737169676563615957555351494745434139373533312927252321191715131197531
(list; graph; listen)
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OFFSET
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2,1
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COMMENT
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Sequence starts with offset 2 since A038395(1)=1 has no prime factor. - M. F. Hasler
Starting with 2nd term each 3rd term is a(3m)=3.
Sequence A089922={3,73,123,817,...} lists the largest odd number (2n-1) used in the terms a(n) which are primes or probable primes.
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FORMULA
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a(n)=A020639[A038395(n)]. - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Sep 16 2007
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EXAMPLE
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n=6: a(6)=1579 because 131197531=1579*83089.
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MATHEMATICA
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s=1; Do[news=FromDigits[Flatten[{IntegerDigits[n], IntegerDigits[s]}]]; fi=FactorInteger[news][[1, 1]]; Print[fi]; s=news, {n, 3, 70, 2}]
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PROGRAM
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(PARI) t=1; for( n=2, 99, print1( factor( eval( t=Str( 2*n-1, t)))[1, 1], ", ")) \- M. F. Hasler, Apr 13 2008
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CROSSREFS
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Cf. A038395, A089922, A104644, A138965.
Sequence in context: A040949 A050101 A107811 this_sequence A128372 A040942 A153072
Adjacent sequences: A109834 A109835 A109836 this_sequence A109838 A109839 A109840
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KEYWORD
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base,nonn
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AUTHOR
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Zak Seidov (zakseidov(AT)yahoo.com), Jul 04 2005
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EXTENSIONS
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More terms from R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Sep 16 2007
Edited by M. F. Hasler (Maximilian.Hasler(AT)gmail.com), Apr 14 2008
Edited by T. D. Noe (noe(AT)sspectra.com), Oct 30 2008
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