A091789 a(1) = 1, a(2) = 2, a(n) = reverse concatenation of two previous terms.

1, 2, 21, 122, 22112, 21122221, 1222211221122, 221122112222112222112, 2112222112222112211222211221122221, 1222211221122221122112222112222112211222211222211221122

Offset

1,2

Comments

Concatenation of any three successive terms is a palindrome.

a(17) with 1597 digits is the first term > 10^1000. - Michael S. Branicky, Jul 01 2022

Links

Michael S. Branicky, Table of n, a(n) for n = 1..16

Maple

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

Prog

(Python)
from itertools import count, islice
def agen():
    s, t = "1", "2"
    yield from [1, 2]
    while True:
        s, t = t, (s+t)[::-1]
        yield int(t)
print(list(islice(agen(), 11))) # Michael S. Branicky, Jul 01 2022

Crossrefs

Cf. A091790.

Sequence in context: A068045 A188530 A178328 * A109789 A136588 A171009

Adjacent sequences: A091786 A091787 A091788 * A091790 A091791 A091792

Keyword

base,nonn

Author

Amarnath Murthy, Feb 18 2004

Extensions

More terms from R. J. Mathar, Jul 26 2007

Status

approved