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A371287
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Numbers whose product of prime indices has exactly two distinct prime factors.
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1
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13, 15, 26, 29, 30, 33, 35, 37, 39, 43, 45, 47, 51, 52, 55, 58, 60, 61, 65, 66, 69, 70, 71, 73, 74, 75, 77, 78, 79, 85, 86, 87, 89, 90, 91, 93, 94, 95, 99, 101, 102, 104, 105, 107, 110, 111, 116, 117, 119, 120, 122, 123, 129, 130, 132, 135, 137, 138, 139, 140
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OFFSET
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1,1
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COMMENTS
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A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.
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LINKS
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FORMULA
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EXAMPLE
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The terms together with their prime indices begin:
13: {6}
15: {2,3}
26: {1,6}
29: {10}
30: {1,2,3}
33: {2,5}
35: {3,4}
37: {12}
39: {2,6}
43: {14}
45: {2,2,3}
47: {15}
51: {2,7}
52: {1,1,6}
55: {3,5}
58: {1,10}
60: {1,1,2,3}
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MATHEMATICA
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Select[Range[100], 2==PrimeNu[Times @@ PrimePi/@First/@If[#==1, {}, FactorInteger[#]]]&]
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CROSSREFS
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Counting divisors (not factors) gives A371127, positions of 2's in A370820.
A001221 counts distinct prime factors.
A003963 gives product of prime indices.
A076610 lists products of primes of prime index.
A355731 counts choices of a divisor of each prime index, firsts A355732.
A355741 counts choices of a prime factor of each prime index.
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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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