How do you build a square from this figure?

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enter image description here

If you can use only this figure, what is the LEAST number of such figures that you can use to build a square ? You can use any isometry to build a square and it must be full from the inside.

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The answer is 4. They form a svastika-like pattern (take one of your shape, and make it rotate around the smallest side, by 90°, and repeat this.

enter image description here

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One can also generalize this; if you kept going with the spiral pattern (placing a strip of 7 squares at the end of the previous strip) for all 4 pieces, you would have a square of 8 units.

It's not hard to show inductively that you could continue this without bound to make arbitrarily large squares with even side length.