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A039915 Smallest k such that k(p-1)-1 is positive and divisible by p where p = n-th prime. 4
3, 2, 4, 6, 10, 12, 16, 18, 22, 28, 30, 36, 40, 42, 46, 52, 58, 60, 66, 70, 72, 78, 82, 88, 96, 100, 102, 106, 108, 112, 126, 130, 136, 138, 148, 150, 156, 162, 166, 172, 178, 180, 190, 192, 196, 198, 210, 222, 226, 228, 232, 238, 240, 250, 256, 262, 268, 270 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also smallest positive k such that the k-th triangular number is divisible by the n-th prime. - Reinhard Zumkeller, Apr 19 2002

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

FORMULA

a(n) = p+(-1)^p. - Juri-Stepan Gerasimov, May 26 2009

EXAMPLE

a(1)=3 because 3(2-1)-1=2 is divisible by 2.

MATHEMATICA

kp[n_]:=Module[{p=Prime[n], k=1}, While[!Divisible[(k(k+1))/2, p], k++]; k]; Array[kp, 60] (* Harvey P. Dale, Jun 12 2012 *)

a[n_] := Prime[n]-1; a[1]=3; Table[a[n], {n, 1, 58}] (* Jean-Fran├žois Alcover, Dec 26 2012, after Juri-Stepan Gerasimov *)

CROSSREFS

Cf. A000217, A039678.

Essentially the same as A006093.

Sequence in context: A240493 A095258 A114745 * A085346 A121861 A060006

Adjacent sequences:  A039912 A039913 A039914 * A039916 A039917 A039918

KEYWORD

nonn

AUTHOR

Felice Russo

EXTENSIONS

Edited and extended by Ray Chandler, Jun 06 2008

STATUS

approved

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Last modified July 25 15:34 EDT 2017. Contains 289795 sequences.