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A318990
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Numbers of the form prime(x) * prime(y) where x divides y.
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20
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4, 6, 9, 10, 14, 21, 22, 25, 26, 34, 38, 39, 46, 49, 57, 58, 62, 65, 74, 82, 86, 87, 94, 106, 111, 115, 118, 121, 122, 129, 133, 134, 142, 146, 158, 159, 166, 169, 178, 183, 185, 194, 202, 206, 213, 214, 218, 226, 235, 237, 254, 259, 262, 267, 274, 278, 289
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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LINKS
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Andrew Howroyd, Table of n, a(n) for n = 1..10000
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EXAMPLE
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The sequence of all dividing pairs (columns) begins:
1 1 2 1 1 2 1 3 1 1 1 2 1 4 2 1 1 3 1 1 1 2 1 1
1 2 2 3 4 4 5 3 6 7 8 6 9 4 8 10 11 6 12 13 14 10 15 16
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MATHEMATICA
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Select[Range[100], And[PrimeOmega[#]==2, Or[PrimePowerQ[#], Divisible@@Reverse[PrimePi/@FactorInteger[#][[All, 1]]]]]&]
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PROG
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(PARI) ok(n)={my(f=factor(n)); bigomega(f)==2 && (#f~==1 || primepi(f[2, 1]) % primepi(f[1, 1]) == 0)} \\ Andrew Howroyd, Oct 26 2018
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CROSSREFS
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A subset of A001358 (semiprimes), squarefree A006881.
The squarefree version is A339005.
The quotient is A358103 = A358104 / A358105.
A000040 lists the primes.
A001222 counts prime indices, distinct A001221.
A003963 multiplies together prime indices.
A056239 adds up prime indices.
A358192/A358193 gives quotients of semiprime indices.
Cf. A000837, A300912, A318991, A318992, A318993.
Cf. A000720, A027751, A032741, A215366, A296150.
Cf. A289508, A289509, A358106.
Sequence in context: A084759 A054395 A142863 * A132435 A108631 A200677
Adjacent sequences: A318987 A318988 A318989 * A318991 A318992 A318993
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KEYWORD
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nonn
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AUTHOR
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Gus Wiseman, Sep 06 2018
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STATUS
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approved
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