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A253385
Numbers divisible by at least three distinct primes whose largest prime power factor is not based on its smallest nor its greatest prime factor.
0
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
OFFSET
1,1
COMMENTS
This sequence contains all unimodal composites (numbers whose list of prime factors is strictly increasing then strictly decreasing).
EXAMPLE
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).
MATHEMATICA
Module[{pfl},
Select[Range[2000],
Function[n, pfl = Power @@@ FactorInteger[n];
1 < First[First[Position[pfl, Max[pfl], 1]]] < Length[pfl]]]]
CROSSREFS
Cf. A057715 (numbers with strictly decreasing prime power factor list).
Sequence in context: A025362 A114826 A332725 * A379093 A332831 A103653
KEYWORD
nonn,easy
AUTHOR
Olivier Gérard, Dec 30 2014
STATUS
approved