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

Author

Zak Seidov, Jul 04 2005

Keywords

• base
• nonn

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.

Examples

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

Links

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

Crossrefs

Cf. A038395, A089922, A104644, A138965.

Programs

Mathematica

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

PARI

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

Formula

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

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