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A253385
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Numbers divisible by at least three distinct primes whose largest prime power factor is not based on its smallest nor its greatest prime factor.
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0
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90, 126, 180, 252, 270, 350, 360, 378, 504, 525, 540, 550, 594, 630, 650, 700, 702, 756, 810, 825, 850, 918, 950, 975, 1026, 1050, 1078, 1080, 1100, 1134, 1150, 1188, 1242, 1260, 1274, 1275, 1300, 1350, 1400, 1404, 1425, 1512, 1575, 1617, 1620, 1650, 1666, 1700, 1725, 1750, 1782, 1836, 1862, 1890, 1900, 1911, 1950
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OFFSET
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1,1
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COMMENTS
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This sequence contains all unimodal composites (numbers whose list of prime factors is strictly increasing then strictly decreasing).
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LINKS
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EXAMPLE
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90 is the first member of this sequence because its prime factor decomposition is 2*3^2*5, using the three smallest primes and 3^2 = 9 is the first power of 3 greater than 5 (and 2).
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MATHEMATICA
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Module[{pfl},
Select[Range[2000],
Function[n, pfl = Power @@@ FactorInteger[n];
1 < First[First[Position[pfl, Max[pfl], 1]]] < Length[pfl]]]]
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CROSSREFS
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Cf. A057715 (numbers with strictly decreasing prime power factor list).
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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