A109837 -id:A109837 - cp's OEIS frontend

A038395 Concatenation of the first n odd numbers in reverse order.

1, 31, 531, 7531, 97531, 1197531, 131197531, 15131197531, 1715131197531, 191715131197531, 21191715131197531, 2321191715131197531, 252321191715131197531, 27252321191715131197531, 2927252321191715131197531, 312927252321191715131197531

Offset: 1

M. I. Petrescu (mipetrescu(AT)yahoo.com)

a(n) starts with the digits of 2n-1. Indices of prime or probable prime terms are 1,2,37,62,409,...: see also A089922. - M. F. Hasler, Apr 13 2008

If n == 0 (mod 3), so is a(n). - Sergey Pavlov, Mar 29 2017

Mihaly Bencze [Beneze] and L. Tutescu, Some Notions and Questions in Number Theory, Sequence 3.

Florentin Smarandache, Sequences of Numbers Involved in Unsolved Problems, arXiv:math/0604019 [math.GM], 2006.

Cf. A089922, A109837, A038394-A038399, A019518, A019519.

Table[FromDigits[Flatten[IntegerDigits/@Join[Reverse[Range[1,n,2]]]]], {n,1,29,2}] (* Harvey P. Dale, Jun 02 2011 *)

t=""; for( n=1,10^3, ( t=eval( Str( 2*n-1,t))) & print(n" "t)) \\ M. F. Hasler, Apr 13 2008

def a(n): return int("".join(map(str, range(2*n-1, 0, -2))))
print([a(n) for n in range(1, 17)]) # Michael S. Branicky, Jan 31 2021

Edited and extended by M. F. Hasler, Apr 13 2008

Edited by T. D. Noe, Oct 30 2008

A138965 Least prime factor of concatenation of first n odd numbers.

Original entry on oeis.org

1, 13, 3, 23, 37, 3, 11617, 5, 3, 135791113151719, 29, 3, 5, 11, 3, 135791113151719212325272931, 17, 3, 7, 13, 3, 131, 5, 3, 11, 25471443030907588399109, 3, 5, 7, 3, 181, 41, 3, 135791113151719212325272931333537394143454749515355575961636567, 19, 3, 40351, 5, 3, 7, 11, 3, 5, 57041, 3, 351269, 11, 3, 135791113151719212325272931333537394143454749515355575961636567697173757779818385878991939597

Offset: 1

M. F. Hasler, Apr 14 2008

Sean A. Irvine, Table of n, a(n) for n = 1..300
F. Smarandache, Sequences of numbers involved in unsolved problems, arXiv:math/0604019

Cf. A019519, A046036, A048847; A109837, A104644.

t=1; for( n=2,99, print1( factor( eval( t=Str( t,2*n-1 )))[1,1], ", "))

A138965(n) = A020639(A019519(n)) (= 3 if n = 0 (mod 3)).

A109840 Largest prime factor of the reverse concatenation of the first n consecutive odd numbers.

Original entry on oeis.org

31, 59, 443, 13933, 2609, 83089, 18937669, 57697, 71615663503, 161768817795401, 4356390089, 4668342004597, 109230695824219, 2770122504473483, 217596035578054625221, 1721572826917, 659160492491

Offset: 1

Zak Seidov, Jul 05 2005

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

Cf. A109837 = smallest prime factor of the reverse concatenation of the first n consecutive odd numbers.

s=1;ss={};Do[news=FromDigits[Flatten[{IntegerDigits[n], IntegerDigits[s]}]];fi=FactorInteger[news][[ -1, 1]];s=news;AppendTo[ss, fi], {n, 3, 60, 2}]; A109840=ss
With[{nn=17},Table[FactorInteger[FromDigits[Flatten[IntegerDigits/@Reverse[Range[1,2n+1,2]]]]][[-1,1]],{n,nn}]] (* Harvey P. Dale, Sep 02 2023 *)

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A038395 Concatenation of the first n odd numbers in reverse order.

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A138965 Least prime factor of concatenation of first n odd numbers.

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A109840 Largest prime factor of the reverse concatenation of the first n consecutive odd numbers.

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