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.
5 = +1(9)-1(3)-1(1) -> +1(9)-1(-3)-1(1) = 11 = a(5)
For n = 0, a(0) = 0. for n = 1, a(1) = -3^0 = 1. for n = 2, a(2) = -3^1 = -3. for n = 3, a(3) = -3^1 + -3^0 = -2. for n = 4, a(4) = -3^2 = 9. for n = 5, a(5) = -3^2 + -3^0 = 10.
(* Returns first 100 numbers in the sequence; assigned to the list, a *)
a = Table[IntegerDigits[x, 2], {x, 0, 100}];
For[i = 1, i <= Length[a], i++,
For[j = 1, j <= Length[a[[i]]], j++,
a[[i]][[j]] = ((a[[i]][[j]])*(-3)^(Length[a[[i]]] - j))
]
];
For[i = 1, i <= Length[a], i++, a[[i]] = Total[a[[i]]]];
a
a(n) = my(b=Vecrev(binary(n))); sum(k=1, #b, b[k]*(-3)^(k-1)); \\ Michel Marcus, Mar 22 2021
a(n) = fromdigits(binary(n),-3) \\ Kevin Ryde, Mar 22 2021
def a(n): return sum((-3)**k for k, b in enumerate(bin(n)[2:][::-1]) if b=='1') print([a(n) for n in range(64)]) # Michael S. Branicky, Mar 23 2021
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