What is the least number of circles you need to draw, such that every cell of an NxN grid is crossed? A circle crosses a grid cell if one of the points on its circumference lies completely inside the cell (on the border doesn't count). Circles cannot lie outside the grid. What are the best answers for N = 1 to 20? Partial answers are welcome.
An earlier puzzle was about N = 8. This puzzle was suggested by Simd.