A280894 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.

1, 2, 3, 23, 19, 773, 13, 25771, 1325771, 85903775257, 2599552664779907, 4078014120856770452857, 770310365288124131869, 47072294862890243358278834974724275093, 8735279620054327668810964663269852011597, 1500540910023123440069025527770773118908806062201

Offset

1,2

Links

Daniel Suteu, Table of n, a(n) for n = 1..21 (terms 1..17 from Michael De Vlieger)

Example

The largest prime factor of concat(1,2) = 12 is 3: then a(3) = 3;
The largest prime factor of concat(2,3) = 23 is 23: then a(4) = 23; etc.

Maple

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);

Mathematica

a = {1, 2}; Do[AppendTo[a, FactorInteger[FromDigits@ Flatten@ Map[IntegerDigits, Take[a, -2]]][[-1, 1]]], {13}]; a (* Michael De Vlieger, Jan 10 2017 *)

Crossrefs

Cf. A034969, A280893.

Sequence in context: A329916 A378139 A085946 * A042585 A143279 A272271

Adjacent sequences: A280891 A280892 A280893 * A280895 A280896 A280897

Keyword

nonn,base

Author

Paolo P. Lava, Jan 10 2017

Extensions

a(16) from Michael De Vlieger, Jan 10 2017

Status

approved