A091789 - OEIS

%I #14 Jul 01 2022 10:10:53

%S 1,2,21,122,22112,21122221,1222211221122,221122112222112222112,

%T 2112222112222112211222211221122221,

%U 1222211221122221122112222112222112211222211222211221122

%N a(1) = 1, a(2) = 2, a(n) = reverse concatenation of two previous terms.

%C Concatenation of any three successive terms is a palindrome.

%C a(17) with 1597 digits is the first term > 10^1000. - _Michael S. Branicky_, Jul 01 2022

%H Michael S. Branicky, <a href="/A091789/b091789.txt">Table of n, a(n) for n = 1..16</a>

%p Rcat := proc(a,b) local resul,digs ; digs := convert(b,base,10) ; resul := 0 ; for k in digs do resul := resul*10+k ; od ; digs := convert(a,base,10) ; for k in digs do resul := resul*10+k ; od ; RETURN(resul) ; end: A091789 := proc(nmax) local a,anxt,n,i,j; a := [1,2] ; for n from 3 to nmax do a := [op(a),Rcat(op(-2,a),op(-1,a))] ; od; RETURN(a) ; end: op(A091789(13)) ; # _R. J. Mathar_, Jul 26 2007

%o (Python)

%o from itertools import count, islice

%o def agen():

%o     s, t = "1", "2"

%o     yield from [1, 2]

%o     while True:

%o         s, t = t, (s+t)[::-1]

%o         yield int(t)

%o print(list(islice(agen(), 11))) # _Michael S. Branicky_, Jul 01 2022

%Y Cf. A091790.

%K base,nonn

%O 1,2

%A _Amarnath Murthy_, Feb 18 2004

%E More terms from _R. J. Mathar_, Jul 26 2007