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A209192 Numbers n such that digital root of n equals largest prime factor of n.

%I A209192 #17 Dec 22 2024 12:18:54
%S A209192 2,3,5,7,12,48,50,70,128,192,196,320,448,500,700,768,1960,2401,3072,
%T A209192 3200,4480,5000,5488,7000,8192,8575,12288,12544,19600,20480,24010,
%U A209192 28672,30625,32000,44800,49152,50000,54880,67228,70000,78125,85750,109375,125440
%N A209192 Numbers n such that digital root of n equals largest prime factor of n.
%H A209192 Donovan Johnson, <a href="/A209192/b209192.txt">Table of n, a(n) for n = 1..1000</a>
%H A209192 PrimeFan, <a href="http://primefan.tripod.com/EsotericIntegerSequences.html">Listing of Esoteric Integer Sequences</a>
%H A209192 PrimeFan, <a href="/A031214/a031214.pdf">Esoteric Integer Sequences</a> [Cached copy]
%p A209192 A010888 := proc(n)
%p A209192     d := (add(d,d=convert(n,base,10))) mod 9 ;
%p A209192     if d = 0 then
%p A209192         return 9;
%p A209192     else
%p A209192         return d;
%p A209192     end if;
%p A209192 end proc:
%p A209192 isA209192 := proc(n)
%p A209192     A010888(n) = A006530(n) ;
%p A209192 end proc:
%p A209192 for n from 2 to 40000 do    if isA209192(n) then   printf("%d,",n) ;    end if; end do: # _R. J. Mathar_, Mar 22 2012
%t A209192 drlpfQ[n_]:=NestWhile[Total[IntegerDigits[#]]&,n,#>9&]==FactorInteger[n][[-1,1]]; Select[Range[2,126000],drlpfQ] (* _Harvey P. Dale_, Sep 26 2024 *)
%K A209192 nonn,base
%O A209192 1,1
%A A209192 _N. J. A. Sloane_, Mar 05 2012