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Numbers n such that digital root of n equals largest prime factor of n.
1

%I #17 Dec 22 2024 12:18:54

%S 2,3,5,7,12,48,50,70,128,192,196,320,448,500,700,768,1960,2401,3072,

%T 3200,4480,5000,5488,7000,8192,8575,12288,12544,19600,20480,24010,

%U 28672,30625,32000,44800,49152,50000,54880,67228,70000,78125,85750,109375,125440

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

%H Donovan Johnson, <a href="/A209192/b209192.txt">Table of n, a(n) for n = 1..1000</a>

%H PrimeFan, <a href="http://primefan.tripod.com/EsotericIntegerSequences.html">Listing of Esoteric Integer Sequences</a>

%H PrimeFan, <a href="/A031214/a031214.pdf">Esoteric Integer Sequences</a> [Cached copy]

%p A010888 := proc(n)

%p d := (add(d,d=convert(n,base,10))) mod 9 ;

%p if d = 0 then

%p return 9;

%p else

%p return d;

%p end if;

%p end proc:

%p isA209192 := proc(n)

%p A010888(n) = A006530(n) ;

%p end proc:

%p 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 drlpfQ[n_]:=NestWhile[Total[IntegerDigits[#]]&,n,#>9&]==FactorInteger[n][[-1,1]]; Select[Range[2,126000],drlpfQ] (* _Harvey P. Dale_, Sep 26 2024 *)

%K nonn,base

%O 1,1

%A _N. J. A. Sloane_, Mar 05 2012