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A215217 First of a pair of sphenic twins, i.e. consecutive integers, each the product of three distinct primes. 6
230, 285, 429, 434, 609, 645, 741, 805, 902, 969, 986, 1001, 1022, 1065, 1085, 1105, 1130, 1221, 1245, 1265, 1309, 1310, 1334, 1406, 1434, 1442, 1462, 1490, 1505, 1533, 1581, 1598, 1605, 1614, 1634, 1729, 1742, 1833, 1885, 1886, 1946, 2013, 2014, 2054, 2085 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

455 is not in a(n), since 455 = 5*7*13 is sphenic, i.e. the number of distinct prime factors is 3, though 456 = 2^3*3*19 has 3 distinct prime factors but is not sphenic, because the number of prime factors is 5 > 3.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

MAPLE

Sphenics:= select(t -> (map(s->s[2], ifactors(t)[2])=[1, 1, 1]), {$1..10000}):

Sphenics intersect map(`-`, Sphenics, 1); # Robert Israel, Aug 13 2014

MATHEMATICA

Select[Range[2500], (PrimeNu[#] == PrimeOmega[#] == PrimeNu[#+1] == PrimeOmega[#+1] == 3)&] (* Jean-Fran├žois Alcover, Apr 11 2014 *)

CROSSREFS

Cf. A007304, A066509, A140077.

Sequence in context: A122269 A171666 A140077 * A211711 A211716 A251445

Adjacent sequences:  A215214 A215215 A215216 * A215218 A215219 A215220

KEYWORD

nonn

AUTHOR

Martin Renner, Aug 06 2012

STATUS

approved

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Last modified June 25 08:34 EDT 2017. Contains 288709 sequences.