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 A215217 Smaller member of a pair of sphenic twins, consecutive integers, each the product of three distinct primes. 10
 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 a term of the sequence, 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 with repetition is 5 > 3. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 MAPLE Sphenics:= select(t -> (map(s->s, ifactors(t))=[1, 1, 1]), {\$1..10000}): Sphenics intersect map(`-`, Sphenics, 1); # Robert Israel, Aug 13 2014 MATHEMATICA Select[Range, (PrimeNu[#] == PrimeOmega[#] == PrimeNu[#+1] == PrimeOmega[#+1] == 3)&] (* Jean-François Alcover, Apr 11 2014 *) SequencePosition[Table[If[PrimeNu[n]==PrimeOmega[n]==3, 1, 0], {n, 2500}], {1, 1}][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 02 2017 *) PROG (Haskell) twinLow [] = [] twinLow [_] = [] twinLow (n : (m : ns))     | m == n + 1 = n : twinLow (m : ns)     | otherwise = twinLow (m : ns) a215217 n = (twinLow a007304_list) !! (n - 1) -- Peter Dolland, May 31 2019 (PARI) is_a033992(n) = omega(n)==3 && bigomega(n)==3 is(n) = is_a033992(n) && is_a033992(n+1) \\ Felix Fröhlich, Jun 10 2019 CROSSREFS Cf. A007304, A066509, A140077. Sequence in context: A171666 A321503 A140077 * A291617 A304389 A211711 Adjacent sequences:  A215214 A215215 A215216 * A215218 A215219 A215220 KEYWORD nonn AUTHOR Martin Renner, Aug 06 2012 STATUS approved

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Last modified January 26 14:08 EST 2020. Contains 331280 sequences. (Running on oeis4.)