A280893 a(n) is the maximum prime factor of the concatenation of all the previous terms, with a(1)=1, a(2)=2.

1, 2, 3, 41, 43, 1063, 5479, 111031, 790000148543, 790000148543, 326139075156576200419624217119, 326139075156576200419624217119, 326139075156576200419624217119, 246787955464079218902570922322710067417716295997334514692275780099917

Offset: 1

Author

Paolo P. Lava, Jan 10 2017

Keywords

• nonn
• base

Examples

			The maximum prime factor of concat(1,2) = 12 is 3, so a(3) = 3;
The maximum prime factor of concat(1,2,3) = 123 is 41, so a(4) = 41; etc.
		

Links

• Hans Havermann, Table of n, a(n) for n = 1..15

Crossrefs

Cf. A280894.

Programs

Maple

with(numtheory): P:= proc(q) local a,b,c,k,n; print(1); print(2); a:=12;for n from 3 to q do b:=ifactors(a)[2]; c:=0; for k from 1 to nops(b) do if b[k][1]>c then c:=b[k][1]; fi; od; a:=a*10^(ilog10(c)+1)+c; print(c); od; end: P(10^2);

Mathematica

a = {1, 2}; Do[AppendTo[a, FactorInteger[FromDigits@ Flatten@ Map[IntegerDigits, a]][[-1, 1]]], {10}]; a (* Michael De Vlieger, Jan 10 2017 *)

Extensions

a(12)-a(13) from Jon E. Schoenfield, Jan 10 2017

a(14)-a(15) from Hans Havermann, Jan 12 2017