Table of n, a(n) for n=1..10.
Puzzling Stack Exchange, Least number of marked symbols
For n = 3, painting only 2 cells would leave an unpainted pentomino, but painting the following 3 cells avoids all unpainted pentominoes:
. . .
. X X
X . .
Cf. A337501, A337502.
Sequence in context: A097431 A123929 A036715 * A310035 A310036 A265046
Adjacent sequences: A337500 A337501 A337502 * A337504 A337505 A337506
Rob Pratt, Aug 29 2020