A170825 - OEIS

%I #22 Jan 15 2022 12:30:20

%S 1,1,1,1,5,1,1,1,1,5,11,1,1,1,5,1,17,1,1,5,1,11,23,1,5,1,1,1,29,5,1,1,

%T 11,17,5,1,1,1,1,5,41,1,1,11,5,23,47,1,1,5,17,1,53,1,55,1,1,29,59,5,1,

%U 1,1,1,5,11,1,17,23,5,71,1,1,1,5,1,11,1,1,5,1,41,83,1,85,1,29,11,89,5,1

%N a(n) is the product of the distinct primes of form 6*k-1 that divide n.

%H Amiram Eldar, <a href="/A170825/b170825.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A007947(A343431(n)) = A343431(A007947(n)). - _Peter Munn_, Jan 15 2022

%p A170825 := proc(n) a := 1 ; for p in numtheory[factorset](n) do if p mod 6 = 5 then a := a*p ; end if ; end do ; a ; end proc: seq(A170825(n),n=1..120) ; # _R. J. Mathar_, Jan 21 2010

%t Table[Times@@Select[Transpose[FactorInteger[n]][[1]],IntegerQ[(#+1)/6]&],{n,100}] (* _Harvey P. Dale_, Nov 01 2013 *)

%o (PARI) a(n) = my(f=factor(n)); for(i=1, #f~, f[i,2] = (f[i,1]%6)==5); factorback(f); \\ _Michel Marcus_, Sep 30 2020

%Y Cf. A007528, A007947, A170819, A343431.

%Y Cf. A140214. - _R. J. Mathar_, Jan 21 2010

%K nonn,mult,easy

%O 1,5

%A _N. J. A. Sloane_, Dec 25 2009, following a suggestion from _Jonathan Vos Post_

%E More terms from _R. J. Mathar_, Jan 21 2010