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A338912
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Lesser prime index of the n-th semiprime.
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43
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1, 1, 2, 1, 1, 2, 2, 1, 3, 1, 2, 1, 3, 1, 2, 1, 4, 2, 3, 2, 1, 1, 3, 2, 1, 4, 1, 3, 1, 2, 4, 2, 1, 3, 1, 2, 3, 1, 4, 5, 1, 2, 2, 4, 1, 2, 1, 5, 3, 1, 3, 1, 2, 4, 1, 6, 2, 1, 2, 3, 5, 1, 2, 1, 4, 3, 1, 5, 2, 1, 3, 4, 1, 2, 6, 1, 3, 2, 6, 2, 5, 1, 4, 1, 3, 2, 1
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OFFSET
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1,3
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COMMENTS
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A semiprime is a product of any two prime numbers. 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 semiprimes are:
2*2, 2*3, 3*3, 2*5, 2*7, 3*5, 3*7, 2*11, 5*5, 2*13, ...
so the lesser prime factors are:
2, 2, 3, 2, 2, 3, 3, 2, 5, 2, ...
with indices:
1, 1, 2, 1, 1, 2, 2, 1, 3, 1, ...
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MATHEMATICA
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Table[Min[PrimePi/@First/@FactorInteger[n]], {n, Select[Range[100], PrimeOmega[#]==2&]}]
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CROSSREFS
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A084126 is the lesser prime factor (not index).
A128301 lists positions of first appearances of each positive integer.
A001221 counts distinct prime indices.
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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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