

A180714


Sum of the x and ycoordinates of a point moving in a clockwise spiral.


2



0, 1, 2, 1, 0, 1, 2, 1, 0, 1, 2, 3, 4, 3, 2, 1, 0, 1, 2, 3, 4, 3, 2, 1, 0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1
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OFFSET

0,3


COMMENTS

A spiral on the simple square grid is constructed starting at (0,0) and walking in the closest selfavoiding clockwise loop: up 1 unit, right 1 unit, down 2 units, left 2 units, up 3 units etc. The step widths in the x coordinate are 0, 1, 0, 2, 0, 3,.. a signed version of A142150; the step widths in the y coordinate are 1, 0, 2, 0, 3,... The x coordinate after n steps (n>=0) is a signed variant of A002265(n+3), namely 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, ..; the y coordinate after n steps is 0, 1, 1, 1, 1, 2, 2, ... (n>=0). The sum of the x and y coordinate after n steps (at corners of the spiral) is c(n) = 0, 1, 2, 0, 2, 1, 4, 0, 4, 1, 6, 0, 6, 1, 8, 0,... with g.f. x*(1+x)/( (x1)*(x^2+1)^2). The current sequence is obtained by recording the sum of the two coordinates at all intermediate positions walking with a stride of 1 along the edges of the spiral, equivalent to showing all interpolating integers between two values of c(n). The first differences a(n+1)a(n) are two 1's, four 1's, six 1's, eight 1's etc, blocks of +1 and 1 with run lengths increasing by 2.  R. J. Mathar, Jan 22 2011


LINKS

Peter Kagey, Table of n, a(n) for n = 0..10000
Image of the spiral


EXAMPLE

Spiral begins at x=0,y=0, then movers uprightdownleftupright...
a(0)=0+0=0, a(1)=1+0=1, a(2)=1+1=2, a(3)=0+1=1, a(4)=1+1=0, a(5)=1+0=1, ...


CROSSREFS

Cf. A065358, A174344.
Sequence in context: A065368 A010751 A194523 * A170959 A170960 A170961
Adjacent sequences: A180711 A180712 A180713 * A180715 A180716 A180717


KEYWORD

sign


AUTHOR

Vladimir Joseph Stephan Orlovsky, Jan 21 2011


STATUS

approved



