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 A075123 #31 Jun 03 2025 02:55:21
%S A075123 1,2,3,6,9,14,17,22,25,30,33,38,41,46,49,54,57,62,65,70,73,78,81,86,
%T A075123 89,94,97,102,105,110,113,118,121,126,129,134,137,142,145,150,153,158,
%U A075123 161,166,169,174,177,182,185,190,193,198,201,206,209,214,217,222,225,230
%N A075123 a(n) is the least positive integer > a(n-1) and a(n) is not 2*a(i)+a(j) for 1<=i<j<n.
%C A075123 a(n) = A047452(n-2) for n > 3 because of first formula. - _Georg Fischer_, Oct 19 2018
%H A075123 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A075123 a(n) = 4n - 10 - (n mod 2), for n>3. - _Ralf Stephan_, Nov 16 2004
%F A075123 a(n) = a(n-1) + a(n-2) - a(n-3) for n > 3. - _Harvey P. Dale_, Oct 28 2012
%F A075123 G.f.: x*(1+x+2*x^3+2*x^4+2*x^5)/((1+x)*(1-x)^2). - _Georg Fischer_, May 15 2019
%t A075123 Join[{1,2,3},Table[4n-10-Mod[n,2],{n,4,60}]] (* or *)
%t A075123 LinearRecurrence[ {1,1,-1},{1,2,3,6,9,14},60] (* _Harvey P. Dale_, Oct 28 2012 *)
%o A075123 (Python)
%o A075123 def A075123(n): return (n-2<<2)-2-(n&1) if n>3 else n # _Chai Wah Wu_, Mar 30 2024
%Y A075123 Cf. A003278, A026471, A033627, A047452, A075122.
%K A075123 nonn,easy
%O A075123 1,2
%A A075123 _Floor van Lamoen_, Sep 02 2002