a(1)..a(6),a(8),a(12) = 0 as these squares cannot be tiled with distinct rectangles with edge lengths that do not divide n. For example for the 8 x 8 square only three rectangles are available with dimensions 3 x 3, 3 x 5, and 5 x 5. All other rectangles have an edge length that divides 8 else leave a space of size 1 or 2 units between its edge and the edge of the square. These gaps cannot be filled as no rectangle can have an edge length of 1 or 2.
a(7) = 360. And example tiling is:
.
+---+---+---+---+---+---+---+
| | | |
+ + + +
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+---+---+---+---+---+ +
| | |
+ + +
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+---+---+---+---+---+---+---+
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+ + +
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+ + +
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+---+---+---+---+---+---+---+
.