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 A106481 #14 Aug 04 2024 19:12:37
%S A106481 0,1,1,3,3,7,5,13,9,19,13,29,17,41,23,49,31,65,37,83,45,95,55,117,63,
%T A106481 137,75,155,87,183,95,213,111,233,127,257,139,293,157,317,173,357,185,
%U A106481 399,205,423,227,469,243,511,263,543,287,595,305,635,329,671,357,729
%N A106481 An Euler phi transform of 1/(1 - x^2).
%F A106481 a(n) = Sum_{k=0..n} phi(n-k+1)*(k mod 2).
%F A106481 Euler transform of period 7 sequence [3,-2,-1,-1,-2,3,0,...].
%F A106481 a(2n) = A049690(n).
%F A106481 a(2n+1) = A099957(n).
%o A106481 (PARI) a(n) = sum(k=0, n, if (k%2, eulerphi(n-k+1))); \\ _Michel Marcus_, Jun 12 2024
%o A106481 (Python)
%o A106481 # uses programs from A002088 and A049690
%o A106481 def A106481(n): return A002088(n+1)-A049690(n+1>>1) if n&1 else A049690(n>>1) # _Chai Wah Wu_, Aug 04 2024
%Y A106481 Cf. A049690, A099957.
%K A106481 easy,nonn
%O A106481 0,4
%A A106481 _Paul Barry_, May 03 2005