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A109837 Smallest prime factor of the reverse concatenation of the first n odd numbers. 3
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; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

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.

LINKS

Table of n, a(n) for n=2..37.

FORMULA

a(n)=A020639[A038395(n)]. - R. J. Mathar, Sep 16 2007

EXAMPLE

n=6: a(6)=1579 because 131197531=1579*83089.

MATHEMATICA

s=1; Do[news=FromDigits[Flatten[{IntegerDigits[n], IntegerDigits[s]}]]; fi=FactorInteger[news][[1, 1]]; Print[fi]; s=news, {n, 3, 70, 2}]

PROG

(PARI) t=1; for( n=2, 99, print1( factor( eval( t=Str( 2*n-1, t)))[1, 1], ", ")) \- M. F. Hasler, Apr 13 2008

CROSSREFS

Cf. A038395, A089922, A104644, A138965.

Sequence in context: A040949 A050101 A107811 * A128372 A040942 A153072

Adjacent sequences:  A109834 A109835 A109836 * A109838 A109839 A109840

KEYWORD

base,nonn

AUTHOR

Zak Seidov, Jul 04 2005

EXTENSIONS

More terms from R. J. Mathar, Sep 16 2007

Edited by M. F. Hasler, Apr 14 2008

Edited by T. D. Noe, Oct 30 2008

STATUS

approved

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Last modified March 22 04:32 EDT 2019. Contains 321406 sequences. (Running on oeis4.)