

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
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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



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)&] (* JeanFrançois 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)
(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



KEYWORD

nonn


AUTHOR



STATUS

approved



