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A345212
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Numbers with equal numbers of prime and semiprime divisors.
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1
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1, 4, 8, 9, 12, 16, 18, 20, 24, 25, 27, 28, 30, 32, 40, 42, 44, 45, 48, 49, 50, 52, 54, 56, 63, 64, 66, 68, 70, 75, 76, 78, 80, 81, 88, 92, 96, 98, 99, 102, 104, 105, 110, 112, 114, 116, 117, 121, 124, 125, 128, 130, 135, 136, 138, 147, 148, 152, 153, 154, 160, 162, 164, 165
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OFFSET
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1,2
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LINKS
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EXAMPLE
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1 has 0 prime divisors and 0 semiprime divisors.
30 is in the sequence since it has 3 prime divisors: {2,3,5} and 3 semiprime divisors {6,10,15}.
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MATHEMATICA
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pdsdQ[n_]:=Module[{divs=Divisors[n]}, Count[divs, _?(PrimeQ[#]&)]==Count[divs, _?(PrimeOmega[ #]==2&)]]; Select[Range[200], pdsdQ] (* Harvey P. Dale, Apr 03 2024 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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