A161925 - OEIS

%I #9 Apr 23 2018 16:17:37

%S 5,6,7,15,24,26,28,30,55,93,99,105,112,175,182,209,276,292,296,300,

%T 304,308,312,399,506,522,539,549,558,644,744,784,832,905,950,1156,

%U 1312,1328,1369,1431,1458,1485

%N Numbers n such that largest prime factor of n is also largest prime factor of n-th composite number.

%H Harvey P. Dale, <a href="/A161925/b161925.txt">Table of n, a(n) for n = 1..1000</a>

%F {n: A006530(n) = A006530(A002808(n))} [From _R. J. Mathar_, May 21 2010]

%t fi[{a_,b_}]:={FactorInteger[a][[-1,1]],FactorInteger[b][[-1,1]]}; Module[ {nn=2000,len,comps},comps=Select[Range[nn],CompositeQ];len=Length[ comps]; Position[fi/@Thread[{Range[len],comps}],_?(#[[1]]==#[[2]]&), 1, Heads->False]]//Flatten (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Apr 23 2018 *)

%K nonn

%O 1,1

%A _Juri-Stepan Gerasimov_, Jun 22 2009, Jun 27 2009

%E Corrected (5, 7, 175 inserted) and extended beyond 312 by _R. J. Mathar_, May 21 2010