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A020331
Numbers whose base-3 representation is the juxtaposition of two identical strings.
8
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
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 *)
KEYWORD
nonn,base
AUTHOR
David W. Wilson, Melia Aldridge (ma38(AT)spruce.evansville.edu)
STATUS
approved