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A338900
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Difference between the two prime indices of the n-th squarefree semiprime.
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28
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1, 2, 3, 1, 2, 4, 5, 3, 6, 1, 7, 4, 8, 5, 2, 6, 9, 10, 3, 7, 11, 1, 12, 4, 13, 8, 2, 9, 14, 5, 15, 10, 6, 16, 3, 17, 11, 12, 4, 18, 13, 19, 1, 7, 20, 8, 21, 14, 5, 22, 15, 23, 16, 9, 2, 24, 17, 25, 6, 10, 26, 3, 18, 27, 11, 7, 28, 19, 1, 29, 12, 20, 2, 21, 4
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OFFSET
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1,2
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COMMENTS
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A squarefree semiprime is a product of any two distinct prime numbers. A prime index of n is a number m such that the m-th prime number divides n. The multiset of prime indices of n is row n of A112798.
Is this sequence an anti-run, i.e., are there no adjacent equal parts? I have verified this conjecture up to n = 10^6. - Gus Wiseman, Nov 18 2020
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LINKS
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FORMULA
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If the n-th squarefree semiprime is prime(x) * prime(y) with x < y, then a(n) = y - x.
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MATHEMATICA
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-Subtract@@PrimePi/@First/@FactorInteger[#]&/@Select[Range[100], SquareFreeQ[#]&&PrimeOmega[#]==2&]
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CROSSREFS
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A176506 is the not necessarily squarefree version.
A338899 has row-differences equal to this sequence.
A338901 gives positions of first appearances.
A001221 counts distinct prime indices.
A004526 counts 2-part partitions, with strict case A140106 (shifted left).
A065516 gives first differences of semiprimes.
A166237 gives first differences of squarefree semiprimes.
Cf. A000040, A056239, A112798, A167171, A320656, A320891, A320894, A320911, A338898, A338905, A338908.
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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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