A109837 Smallest prime factor of the reverse concatenation of the first n odd numbers.

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

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

Sean A. Irvine, Table of n, a(n) for n = 2..103

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 A334061 A040942

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