A020331 - OEIS

%I #23 Apr 06 2021 10:00:29

%S 4,8,30,40,50,60,70,80,252,280,308,336,364,392,420,448,476,504,532,

%T 560,588,616,644,672,700,728,2214,2296,2378,2460,2542,2624,2706,2788,

%U 2870,2952,3034,3116,3198,3280,3362,3444,3526,3608,3690,3772,3854,3936,4018

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

%H Amiram Eldar, <a href="/A020331/b020331.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = n*3^floor(log_3(n)+1) + n. - _Ilya Gutkovskiy_, Jan 26 2018

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

%t 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 *)

%t a[n_] := n + n*3^Floor[Log[3, n] + 1]; Array[a, 50] (* _Amiram Eldar_, Apr 06 2021 *)

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

%K nonn,base

%O 1,1

%A _David W. Wilson_, Melia Aldridge (ma38(AT)spruce.evansville.edu)