A020331 Numbers whose base-3 representation is the juxtaposition of two identical strings.

4, 8, 30, 40, 50, 60, 70, 80, 252, 280, 308, 336, 364, 392, 420, 448, 476, 504, 532, 560, 588, 616, 644, 672, 700, 728, 2214, 2296, 2378, 2460, 2542, 2624, 2706, 2788, 2870, 2952, 3034, 3116, 3198, 3280, 3362, 3444, 3526, 3608, 3690, 3772, 3854, 3936, 4018

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Formula

a(n) = n*3^floor(log_3(n)+1) + n. - Ilya Gutkovskiy, Jan 26 2018

Example

50_10 = 1212_3. - Jon E. Schoenfield, Feb 11 2021

Mathematica

b3iQ[n_]:=Module[{idn3=IntegerDigits[n, 3], len}, len=Length[idn3]; EvenQ[ len] && Take[idn3, len/2]==Take[idn3, -len/2 ]]; Select[Range[5000], b3iQ] (* Harvey P. Dale, Feb 08 2015 *)
a[n_] := n + n*3^Floor[Log[3, n] + 1]; Array[a, 50] (* Amiram Eldar, Apr 06 2021 *)

Crossrefs

Cf. A020330, A020332, A020333, A020334, A020335, A020336, A020337, A020338.

Sequence in context: A270522 A372384 A317583 * A248476 A082595 A262154

Adjacent sequences: A020328 A020329 A020330 * A020332 A020333 A020334

Keyword

nonn,base

Author

David W. Wilson, Melia Aldridge (ma38(AT)spruce.evansville.edu)

Status

approved