A209191 Numbers n such that digital root of n equals smallest prime factor of n.

2, 3, 5, 7, 20, 21, 38, 39, 56, 57, 74, 75, 92, 93, 95, 110, 111, 128, 129, 133, 146, 147, 164, 165, 182, 183, 185, 200, 201, 218, 219, 236, 237, 254, 255, 259, 272, 273, 275, 290, 291, 308, 309, 326, 327, 344, 345, 362, 363, 365, 380, 381, 398

Offset: 1

Author

N. J. A. Sloane, Mar 05 2012

Keywords

• nonn
• base

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000
• PrimeFan, Listing of Esoteric Integer Sequences
• PrimeFan, Esoteric Integer Sequences [Cached copy]

Programs

Mathematica

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

Extensions

Prior Mathematica program replaced by Harvey P. Dale, Nov 11 2017