Table of n, a(n) for n=1..10.
For n = 4, painting only 5 cells would leave an unpainted tetromino, but painting the following 6 cells avoids all unpainted tetrominoes:
. . X .
X X . X
. X . .
Sequence in context: A143981 A031940 A007187 * A082004 A112773 A070885
Adjacent sequences: A337499 A337500 A337501 * A337503 A337504 A337505
Rob Pratt, Aug 29 2020