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A332583
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Label only the prime-numbered position cells of the infinite 2D square lattice with the square spiral (or Ulam spiral), starting with 1 at the center; sequence lists primes that are visible from square 1.
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2
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2, 3, 5, 7, 19, 23, 29, 41, 47, 59, 61, 67, 71, 79, 83, 89, 97, 103, 107, 109, 113, 131, 137, 149, 167, 173, 179, 181, 191, 193, 199, 223, 227, 229, 239, 251, 263, 271, 277, 283, 293, 311, 317, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 409, 419, 433, 439, 443, 449, 457, 461, 467, 479, 487, 491, 499, 503
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OFFSET
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1,1
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COMMENTS
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Any grid point labeled with a prime number and with coordinates (x,y) relative to the central grid point, which is numbered 1, and where the greatest common divisor (gcd) of |x| and |y| equals 1 will be visible from the central point. Grid points where gcd(|x|,|y|) > 1 may have another prime grid point directly between it and the central point and will thus not be visible.
For a square spiral of size 10001 by 10001, slightly over 100 million numbers, a total of 5762536 primes are present, of which 4811013 are visible. This gives a ratio of visible primes to all primes of about 0.835.
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LINKS
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EXAMPLE
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The 2D grid is shown below. The primes that are blocked from the central 1 square are in parentheses; these all have another prime number directly between their position and the central square.
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-------------61-------59------+
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(37)---------------------(31) |
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| (17)--------------(13) | |
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| | 5--------3 | 29 |
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| 19 | 1----2 (11) | (53)
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41 | 7------------+ | |
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| +-------23-----------+ |
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(43)-------------47-----------+
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a(1) = 2 to a(4) = 7 are all primes adjacent to the central 1 point, thus all are visible from that square.
a(5) = 19 as primes 11,13,17 are blocked from the central 1 point by points with prime numbers 2,3,5 respectively.
a(14) = 79 as although the point 79 has relative coordinates of (2,-4) from the central square, gcd(|2|,|-4|) = 2, there is no other prime at coordinate (1,-2), thus it is visible. This square is not visible from the central square when nonprime points are also considered in the spiral.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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