A348690 - OEIS

%I #19 Dec 30 2024 01:18:12

%S 0,1,1,0,0,-1,-1,0,-2,-1,-1,2,-2,1,1,2,-4,1,1,4,0,3,3,0,-2,3,3,2,2,1,

%T 1,-2,-4,5,5,4,4,3,3,-4,2,3,3,-2,2,-3,-3,-2,0,5,5,0,4,-1,-1,-4,2,-1,

%U -1,-2,-2,-3,-3,2,0,9,9,0,8,-1,-1,-8,6,-1,-1,-6,-2

%N For any nonnegative number n with binary expansion Sum_{k >= 0} b_k * 2^k, a(n) is the real part of f(n) = Sum_{k >= 0} b_k * (i^Sum_{j = 0..k-1} b_j) * (1+i)^k (where i denotes the imaginary unit); sequence A348691 gives the imaginary part.

%C The function f defines a bijection from the nonnegative integers to the Gaussian integers.

%C The function f has similarities with A065620; here the nonzero digits in base 1+i cycle through powers of i, there nonzero digits in base 2 cycle through powers of -1.

%C If we replace 1's in binary expansions by powers of i from left to right (rather than right to left as here), then we obtain the Lévy C curve (A332251, A332252).

%H Rémy Sigrist, <a href="/A348690/b348690.txt">Table of n, a(n) for n = 0..8191</a>

%H Chandler Davis and Donald Knuth, Number representations and Dragon Curves I, Journal of Recreational Mathematics, volume 3, number 2 (April 1970), pages 66-81. Reprinted in Donald E. Knuth, <a href="http://www-cs-faculty.stanford.edu/~uno/fg.html">Selected Papers on Fun and Games</a>, CSLI Publications, 2011, pages 571-614.

%H Chandler Davis and Donald E. Knuth, <a href="/A005811/a005811.pdf">Number Representations and Dragon Curves</a>, Journal of Recreational Mathematics, volume 3, number 2, April 1970, pages 66-81, and number 3, July 1970, pages 133-149. [Cached copy, with permission]

%H Rémy Sigrist, <a href="/A348690/a348690.png">Colored representation of f(n) for n < 2^18 in the complex plane</a> (the hue is function of n)

%H Rémy Sigrist, <a href="/A348690/a348690_1.png">Colored representation of f(n) for n < 2^18 in the complex plane</a> (the color is function of A000120(n) mod 4)

%H Rémy Sigrist, <a href="/A348691/a348691.png">Colored representation of f(n) for n < 2^18 in the complex plane</a> (the color is function of the binary length of n, A070939(n))

%F a(2^k) = A146559(k) for any k >= 0.

%o (PARI) a(n) = { my (v=0, o=0, x); while (n, n-=2^x=valuation(n, 2); v+=I^o * (1+I)^x; o++); real(v) }

%Y See A332251 for a similar sequence.

%Y Cf. A000120, A065620, A146559, A348691, A332251, A332252.

%K sign,look,base

%O 0,9

%A _Rémy Sigrist_, Oct 29 2021