Friday, 1 January 2010

recreational mathematics - Walking to infinity on the primes: The prime-spiral moat problem

It is an unsolved problem to decide if it is possible to "walk to infinity" from the origin
with bounded-length steps, each touching a Gaussian prime as a stepping stone.
The paper by Ellen Gethner, Stan Wagon, and Brian Wick,
"A Stroll through the Gaussian Primes"
(American Mathematical Monthly, 105: 327-337 (1998))
discusses this Gaussian moat problem and proves that steps of length <sqrt26 are
insufficient. Their result was improved to sqrt36 in 2005.



My question is:




Is the analogous question easier for the
prime spiral (a.k.a. Ulam spiral)—Can one walk to infinity using bounded-length steps
touching only the spiral coordinates of primes?




What little I know of prime gaps
suggest that should be easier to walk to infinity.
For example, the first gap of 500 does not occur until about 1012
(more precisely, 499 and 303,371,455,241).



I ask this primarily out of curiosity, and have tagged it 'recreational.'



Edit1. In light of Gjergji's remarks below, I have tagged this as an open problem.



Edit2'.
Just for fun, I computed which primes are reachable on a small portion of the spiral,
for step distances dle3 (left below) and dle4 (right below);
red=reached, blue=not reached.
The former does not reach 83, the 23rd prime blue dot barely discernable at spiral coordinates (5,-3);
the latter does not reach 5087, the 680th prime blue dot at
spiral coordinates (36,10).

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