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EXAMPLE
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a(1) = 3 because there are 3 one-digit semiprimes with semiprime digits: 4,6,9.
a(2) = 4 because there are 4 two-digit relevant semiprimes: 46,49,69,94.
a(3) = 8 because there are 8 three-digit relevant semiprimes: 446,466,469,649,669,694,699,949.
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