# How many cells can a planar curve of length $L$ touch at most?

Let $$\mathbf{f}:[0,1]\to\mathbb{R}^2$$ be a continuous function and suppose the curve parametrized by $$\mathbf{f}$$ has finite length $$\leq L$$, i.e., all finite-piece inscribed polylines have total length $$\leq L$$. What is the maximum number of cells it can touch? By that, I mean the number $$N=\left|\left\{(x,y)\in\mathbb{Z}^2:\mathbf{f}([0,1])\cap\left([x,x+1]\times[y,y+1]\right)\neq\varnothing\right\}\right|.$$

It is simple to prove $$N=\mathcal{O}(L)$$, say, $$N\leq 6L+6$$ by considering how much length is required to reach 7 cells. (Citing a friend, the coefficient can be improved to $$1+\sqrt2$$.)

Some fiddling on the scratch paper suggests that the optimum strategy is to take diagonals of the cells (except perhaps taking grid-lines at the ends), giving $$N\geq\frac{3}{\sqrt2}L-c$$ for some constant $$c$$.

Is the above strategy the optimum? How can we prove $$N\leq\frac{3}{\sqrt2}L+c'$$ for some constant $$c'$$? Or even just $$N\leq\frac{3}{\sqrt2}L+o(L)$$?

Extension. How about higher-dimensional spaces? Is the optimum still taking diagonals of length $$\sqrt2$$? (Taking diagonals of length $$\sqrt{k}>\sqrt2$$ is suboptimal.)

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