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A107342
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Semiprimes with semiprime digits (digits 4, 6, 9 only).
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16
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4, 6, 9, 46, 49, 69, 94, 446, 466, 469, 649, 669, 694, 699, 949, 4449, 4469, 4499, 4666, 4694, 4699, 4946, 6499, 6646, 6649, 6694, 6999, 9446, 9449, 9466, 9469, 9946, 9969, 44494, 44669, 44949, 44966, 44969, 44999, 46469, 46666, 46946, 46969, 46994
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OFFSET
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1,1
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COMMENTS
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Numbers n such that all digits of n are elements of A001358 and n is an element of A001358.
Numbers n such that n is an element of A107665 and n is an element of A001358.
Conjecture: almost all terms (asymptotic density 1) end with 9 and have either 3k+1 or 3k+2 occurrences of the digit 4 for some nonnegative k. (Otherwise they'd be divisible by 2 or 3 and these semiprimes would be expected to be rare; the sequence is too thin to prove this directly.) - Charles R Greathouse IV, Nov 12 2021
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LINKS
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EXAMPLE
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4 = 2^2
6 = 2 * 3
9 = 3^2
46 = 2 * 23
49 = 7^2
69 = 3 * 23
94 = 2 * 47
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MATHEMATICA
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fQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2 && Union[ Join[{4, 6, 9}, IntegerDigits[n]]] == {4, 6, 9}; Select[ Range[ 47000], fQ[ # ] &] (* Robert G. Wilson v, May 27 2005 *)
Flatten[Table[Select[FromDigits/@Tuples[{4, 6, 9}, n], PrimeOmega[#]==2&], {n, 5}]] (* Harvey P. Dale, Jun 14 2015 *)
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PROG
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CROSSREFS
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KEYWORD
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base,easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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