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 A137469 #12 Aug 10 2025 04:19:02
%S A137469 1,5,7,8,9,11,16,21,25,27,28,29,31,35,37,38,39,40,42,43,44,45,47,48,
%T A137469 49,51,55,57,58,59,61,66,71,76,80,82,83,84,86,91,96,101,105,107,108,
%U A137469 109,111,116,121,125,127,128,129,131,135,137,138,139,140,142,143,144,145
%N A137469 Numbers with an odd number of 1's in base 5 expansion.
%C A137469 This is to A000069 (Odious numbers: odd number of 1's in binary expansion) as A007091 (Numbers in base 5) is to A007088 (Numbers written in base 2). Note that odd number of 1's in base 3 expansion is simply the odd numbers.
%H A137469 <a href="/index/Ar#5-automatic">Index entries for 5-automatic sequences</a>.
%e A137469 a(18) = 40 (base 10) is in the sequence because 40 (base 10) = 130 (base 5) which has an odd number (1) of 1's ; whereas 41 is not in the sequence because 41 (base 10) = 131 (base 5) has an even number (2) of 1's.
%p A137469 isA137469 := proc(n) local a,d ; a := 0 ; for d in convert(n,base,5) do if d =1 then a := a+1 ; fi ; od; RETURN( a mod 2 = 1 ) ; end: for n from 1 to 400 do if isA137469(n) then printf("%d,",n) ; fi ; od: # _R. J. Mathar_, Apr 21 2008
%t A137469 Select[Range[200], OddQ[DigitCount[ #, 5][[1]]] &] (* _Stefan Steinerberger_, Apr 21 2008 *)
%Y A137469 Cf. A000069, A007088, A007091.
%K A137469 base,easy,nonn
%O A137469 1,2
%A A137469 _Jonathan Vos Post_, Apr 20 2008
%E A137469 More terms from _R. J. Mathar_ and _Stefan Steinerberger_, Apr 21 2008