A209191 - OEIS

%I #16 Sep 17 2025 20:47:43

%S 2,3,5,7,20,21,38,39,56,57,74,75,92,93,95,110,111,128,129,133,146,147,

%T 164,165,182,183,185,200,201,218,219,236,237,254,255,259,272,273,275,

%U 290,291,308,309,326,327,344,345,362,363,365,380,381,398,399,416,417

%N Numbers k such that digital root of k equals smallest prime factor of k.

%C Numbers k such that A010888(k) = A020639(k). - _Jason Yuen_, Sep 17 2025

%H Harvey P. Dale, <a href="/A209191/b209191.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]

%t drspfQ[n_]:=NestWhile[Total[IntegerDigits[#]]&,n,#>9&] == FactorInteger[ n][[1,1]]; Select[Range[2,400],drspfQ] (* _Harvey P. Dale_, Nov 11 2017 *)

%Y Cf. A010888, A020639.

%K nonn,base

%O 1,1

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

%E Prior Mathematica program replaced by _Harvey P. Dale_, Nov 11 2017