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 A065368 #24 Jul 19 2024 08:48:11
%S A065368 0,1,2,-1,0,1,-2,-1,0,1,2,3,0,1,2,-1,0,1,2,3,4,1,2,3,0,1,2,-1,0,1,-2,
%T A065368 -1,0,-3,-2,-1,0,1,2,-1,0,1,-2,-1,0,1,2,3,0,1,2,-1,0,1,-2,-1,0,-3,-2,
%U A065368 -1,-4,-3,-2,-1,0,1,-2,-1,0,-3,-2,-1,0,1,2,-1,0,1,-2,-1,0,1,2,3,0,1,2,-1,0,1,2,3,4,1,2,3,0,1,2,3,4,5,2,3
%N A065368 Alternating sum of ternary digits in n. Replace 3^k with (-1)^k in ternary expansion of n.
%C A065368 Notation: (3)[n](-1).
%C A065368 Fixed point of the morphism 0 -> 0,1,2; 1 -> -1,0,1; 2 -> -2,-1,0; ...; n -> -n,-n+1,-n+2. - _Philippe Deléham_, Oct 22 2011
%F A065368 a(n) = Sum_{k>=0} A030341(n,k)*(-1)^k. - _Philippe Deléham_, Oct 22 2011.
%F A065368 G.f. A(x) satisfies: A(x) = x * (1 + 2*x) / (1 - x^3) - (1 + x + x^2) * A(x^3). - _Ilya Gutkovskiy_, Jul 28 2021
%e A065368 15 = +1(9)+2(3)+0(1) -> +1(+1)+2(-1)+0(+1) = -1 = a(15).
%o A065368 (Python)
%o A065368 from sympy.ntheory.digits import digits
%o A065368 def a(n):
%o A065368 return sum(bi*(-1)**k for k, bi in enumerate(digits(n, 3)[1:][::-1]))
%o A065368 print([a(n) for n in range(104)]) # _Michael S. Branicky_, Jul 28 2021
%o A065368 (Python)
%o A065368 from sympy.ntheory import digits
%o A065368 def A065368(n): return sum((0, 1, 2, -1, 0, 1, -2, -1, 0)[i] for i in digits(n,9)[1:]) # _Chai Wah Wu_, Jul 19 2024
%Y A065368 Cf. A030341, A053735, A065359, A065364.
%K A065368 base,easy,sign
%O A065368 0,3
%A A065368 _Marc LeBrun_, Oct 31 2001
%E A065368 Initial 0 added by _Philippe Deléham_, Oct 22 2011