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

Author

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

Keywords

• nonn
• base

Examples

			50_10 = 1212_3. - _Jon E. Schoenfield_, Feb 11 2021
		

Links

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

Crossrefs

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

Programs

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 *)

Formula

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