A075020 a(1) = 1; for n>1, a(n) = the smallest prime divisor of the number C(n) formed from the reverse concatenation of 1,2,3,... up to n.

1, 3, 3, 29, 3, 3, 19, 3, 3, 7, 3, 3, 17, 3, 3, 23, 3, 3, 17, 3, 3, 13, 3, 3, 11, 3, 3, 23, 3, 3, 7, 3, 3, 89, 3, 3, 29, 3, 3, 11, 3, 3, 52433, 3, 3, 23, 3, 3, 71, 3, 3, 7, 3, 3

Offset: 1

Author

Amarnath Murthy, Sep 01 2002

Keywords

• base
• nonn

Examples

			a(4)= 29, 29 is the smallest prime divisor of 4321 =29*149
		

Crossrefs

Cf. A075019.

Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138960, A138961, A138962.

Programs

Mathematica

b = {}; a = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[Length[w] - k + 1]]], {k, 1, Length[w]}]; p = FromDigits[Reverse[a]]; AppendTo[b, First[First[FactorInteger[p]]]], {n, 1, 21}]; b (* Artur Jasinski, Apr 04 2008 *)

Extensions

More terms from Sascha Kurz, Jan 03 2003