30. (a) The 1/2 x 1/2 subsquare with address (1) is the largest gap.

(b) The 1/4 x 1/4 subsquare with address (2,1) is the largest gap.

(c) The 1/8 x 1/8 subsquare with address (3,2,1) is the largest gap.

(d) If the only forbidden sequence is a string of n characters, then the largest gap
is a 1/2^{n} x 1/2^{n} subsquare.

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