This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A026254 #10 Jan 08 2018 21:01:38 %S A026254 1,1,2,2,3,4,3,5,4,6,5,7,8,6,9,7,10,8,11,12,9,13,10,14,15,11,16,12,17, %T A026254 13,18,19,14,20,15,21,16,22,23,17,24,18,25,19,26,27,20,28,21,29,30,22, %U A026254 31,23,32,24,33,34,25,35,26,36,27,37,38,28 %N A026254 a(n) = j if n = [ j*sqrt(3) ], else a(n) = k if n = [ (k/2)*(3 + sqrt(3)) ]. %C A026254 Every positive integer occurs exactly twice. a(n) is the parent of n in the tree A178528 generated by the Beatty sequence of sqrt(3). %H A026254 Robert Israel, <a href="/A026254/b026254.txt">Table of n, a(n) for n = 1..10000</a> %p A026254 N:= 100: # to get a(1)..a(N) %p A026254 for j from 1 do m:= floor(j*sqrt(3)); if m > N then break fi; A[m]:= j od: %p A026254 for k from 1 do m:= floor(k/2*(3+sqrt(3))); if m > N then break fi; A[m]:= k od: %p A026254 seq(A[i],i=1..N); # _Robert Israel_, Jan 08 2018 %Y A026254 Cf. A178528, A022838, A054406. %K A026254 nonn %O A026254 1,3 %A A026254 _Clark Kimberling_ %E A026254 Comment by _Clark Kimberling_, Dec 24 2010