

A049486


Maximum length of noncrossing path on n X n square lattice.


3



1, 4, 10, 21, 34, 53, 74, 101, 130, 165, 202, 245, 290, 341, 394, 453, 514, 581, 650, 725, 802, 885, 970, 1061, 1154, 1253, 1354, 1461, 1570, 1685, 1802, 1925, 2050, 2181, 2314, 2453, 2594, 2741, 2890, 3045, 3202
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OFFSET

1,2


COMMENTS

One can reuse a point, but not an edge.
From Hugo van der Sanden, Sep 27 2005: (Start)
The n X n square has #(n) = 2n(n+1) line segments and v_o(n) = 4(n1) vertices of odd degree. The network is traversable only when v_o(n) <= 2. When not, we must lose sufficient line segments to reduce v_o() to 2.
For odd n >= 3, we have an even number of odd vertices along each edge; we can pair them up and lose the single line segment joining each pair except for our start/end points. So in this case we have a(n) = #(n)  (v_o(n)  2)/2 = 2n^2 + 3.
For even n >= 2, we have an odd number of odd vertices along each edge. The best we can do is start and end at the two vertices adjacent to one corner; pairing up the rest allows us to lose a single line segment for each of the remaining pairs except for the two vertices adjacent to the diagonally opposite corner, for which we must lose 2 line segments. So in this case we have a(n) = #(n)  ((v_o(n)  4)/2 + 2) = 2n^2 + 2.
For n < 2, we have no vertices of odd degree, so we cannot save a segment by starting and ending on a pair of them. So we can specify the exact function using a couple of characteristic functions: a(n) = 2n^2 + 1 + c(n > 1) + c(n odd) (I'm assuming a(0) = 1 here, on the grounds that there is a single zerolength path in that case.)
Note that the theoretical maximum is always achievable, e.g. using Fleury's algorithm. (End)


LINKS

Table of n, a(n) for n=1..41.
Index entries for linear recurrences with constant coefficients, signature (2, 0, 2, 1).


FORMULA

From Colin Barker, May 02 2013: (Start)
Conjecture: a(n) = (9 + (1)^n  8*n + 4*n^2)/2 for n > 2.
a(n) = 2*a(n1)  2*a(n3) + a(n4) for n > 6.
G.f.: x*(x^5  x^4 + 3*x^3 + 2*x^2 + 2*x + 1) / ((x1)^3*(x+1)). (End)


MATHEMATICA

Join[{1, 4}, LinearRecurrence[{2, 0, 2, 1}, {10, 21, 34, 53}, 40]] (* Harvey P. Dale, Aug 21 2013 *)


CROSSREFS

Cf. A049487.
Sequence in context: A301127 A009919 A008042 * A301130 A008139 A301126
Adjacent sequences: A049483 A049484 A049485 * A049487 A049488 A049489


KEYWORD

nonn,walk,nice


AUTHOR

Arlin Anderson (starship1(AT)gmail.com)


EXTENSIONS

More terms from Hugo van der Sanden, Sep 27 2005


STATUS

approved



