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I am posting the 11 pieces solution shown in the article cited by Michael (it is not freely available online).

This is the smallest known number of pieces. Some remarks:

• The question is open from 4 to 10 for 5, 7, or 9 pieces. Get your pencils!
• Everything so far is with polyominoes. Any suggestion with more complicated shapes?
• Unlike the other solution I posted, this one cannot be resized along the $x$ or $y$ axis.
1

I am posting the 11 pieces solution shown in the article cited by Michael (it is not freely available online).

This is the smallest known number of pieces. Some remarks:

• The question is open from 4 to 10 pieces. Get your pencils!
• Everything so far is with polyominoes. Any suggestion with more complicated shapes?
• Unlike the other solution I posted, this one cannot be resized along the $x$ or $y$ axis.