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%I #23 Jul 07 2022 16:36:42
%S 1,2,3,23,19,773,13,25771,1325771,85903775257,2599552664779907,
%T 4078014120856770452857,770310365288124131869,
%U 47072294862890243358278834974724275093,8735279620054327668810964663269852011597,1500540910023123440069025527770773118908806062201
%N a(n) is the largest prime factor of the concatenation of a(n-2) and a(n-1), with a(1)=1 and a(2)=2.
%H Daniel Suteu, <a href="/A280894/b280894.txt">Table of n, a(n) for n = 1..21</a> (terms 1..17 from Michael De Vlieger)
%e The largest prime factor of concat(1,2) = 12 is 3: then a(3) = 3;
%e The largest prime factor of concat(2,3) = 23 is 23: then a(4) = 23; etc.
%p with(numtheory): P:= proc(q) local a,b,c,d,k,n; print(1); print(2); d:=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:=d*10^(ilog10(c)+1)+c; d:=c; print(c); od; end: P(10^2);
%t a = {1, 2}; Do[AppendTo[a, FactorInteger[FromDigits@ Flatten@ Map[IntegerDigits, Take[a, -2]]][[-1, 1]]], {13}]; a (* _Michael De Vlieger_, Jan 10 2017 *)
%Y Cf. A034969, A280893.
%K nonn,base
%O 1,2
%A _Paolo P. Lava_, Jan 10 2017
%E a(16) from _Michael De Vlieger_, Jan 10 2017
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