The On-Line Encyclopedia of Integer Sequences (OEIS)

Revision History for A115411 (Bold, blue-underlined text is an addition; faded, red-underlined text is a ~~deletion~~.)
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A115411

a(n) = least k such that semiprime(n) divides k-th triangular number.
(history; published version)

#14 by Harvey P. Dale at Tue Jul 09 17:11:30 EDT 2024

STATUS

editing

approved

#13 by Harvey P. Dale at Tue Jul 09 17:11:27 EDT 2024

MATHEMATICA

Module[{nn=500, trno, smpr, k}, trno=Accumulate[Range[nn]]; smpr=Select[Range[ nn], PrimeOmega[ #] == 2&]; Table[ k= SelectFirst[ trno, Mod[ #, smpr[[n]]]==0&]; (Sqrt[8k+1]-1)/2, {n, Length[ smpr]}]] (* Harvey P. Dale, Jul 09 2024 *)

STATUS

approved

editing

#12 by Harvey P. Dale at Tue Jul 09 17:11:05 EDT 2024

STATUS

editing

approved

#11 by Harvey P. Dale at Tue Jul 09 17:11:02 EDT 2024

MATHEMATICA

Module[{nn=500, trno, smpr, k}, trno=Accumulate[Range[nn]]; smpr=Select[Range[ nn], PrimeOmega[#] == 2&]; Table[ k= SelectFirst[ trno, Mod[ #, smpr[[n]]]==0&]; (Sqrt[8k+1]-1)/2, {n, Length[smpr]}]] (* Harvey P. Dale, Jul 09 2024 *)

STATUS

approved

editing

#10 by N. J. A. Sloane at Thu Oct 31 12:17:37 EDT 2013

MATHEMATICA

a = Select[Range@215, Plus @@ Last /@ FactorInteger@# == 2 &]; f[n_] := Block[{k = 1}, While[ Mod[k(k + 1)/2, a[[n]]] > 0, k++ ]; k]; Array[f, 71] (* _Robert G. Wilson v _ *)

Discussion

Thu Oct 31

12:17

OEIS Server: https://oeis.org/edit/global/2036

#9 by Charles R Greathouse IV at Sat Jul 14 11:41:07 EDT 2012

EXTENSIONS

More terms from _Robert G. Wilson v (rgwv(at)rgwv.com), _, May 01 2006

Discussion

Sat Jul 14

11:41

OEIS Server: https://oeis.org/edit/global/1816

#8 by Russ Cox at Fri Mar 30 18:40:36 EDT 2012

AUTHOR

_Jonathan Vos Post (jvospost3(AT)gmail.com), _, Mar 08 2006

Discussion

Fri Mar 30

18:40

OEIS Server: https://oeis.org/edit/global/228

#7 by N. J. A. Sloane at Fri Feb 27 03:00:00 EST 2009

EXAMPLE

a(1) = 7 because SP(1) = semiprime(1) = 4, Triangular number T(7) = 7*(7+1)/2 = 28, and 7 divides 28.

KEYWORD

easy,nonn,new

#6 by N. J. A. Sloane at Fri Jan 09 03:00:00 EST 2009

KEYWORD

easy,nonn,new

AUTHOR

Jonathan Vos Post (jvospost2jvospost3(AT)yahoogmail.com), Mar 08 2006

#5 by N. J. A. Sloane at Sat Nov 10 03:00:00 EST 2007

NAME

a(n) = least k such that semiprime(n) divides kth k-th triangular number.

KEYWORD

easy,nonn,new