

A210615


Least semiprime dividing n, or 0 if no semiprime divides n.


3



0, 0, 0, 4, 0, 6, 0, 4, 9, 10, 0, 4, 0, 14, 15, 4, 0, 6, 0, 4, 21, 22, 0, 4, 25, 26, 9, 4, 0, 6, 0, 4, 33, 34, 35, 4, 0, 38, 39, 4, 0, 6, 0, 4, 9, 46, 0, 4, 49, 10, 51, 4, 0, 6, 55, 4, 57, 58, 0, 4, 0, 62, 9, 4, 65, 6, 0, 4, 69, 10, 0, 4, 0, 74, 15, 4, 77, 6
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OFFSET

1,4


COMMENTS

Roughly analogous to Least Prime Factor A020639 but with semiprimes rather than primes.


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000


FORMULA

a(n) = min {k such that kn and k in A001358} else 0 if there exists no such k.
a(p) = 0 iff p in A008578.  Alois P. Heinz, Mar 28 2012


EXAMPLE

a(24) = 4 because 24 is divisible by the semiprimes {4,6} of which 4 is the smallest.


MATHEMATICA

Table[If[PrimeQ[n]  n < 2, 0, f = FactorInteger[n]; If[f[[1, 2]] > 1, f[[1, 1]]^2, f[[1, 1]]*f[[2, 1]]]], {n, 100}] (* T. D. Noe, Mar 24 2012 *)
Flatten[Table[Select[Divisors[n], PrimeOmega[#]==2&, 1], {n, 80}]/.{}>{0}] (* Harvey P. Dale, Dec 07 2012 *)


CROSSREFS

Cf. A001358, A008578, A020639, A088739 (this sequence without the zeros).
Sequence in context: A125961 A016681 A210625 * A179312 A076290 A198224
Adjacent sequences: A210612 A210613 A210614 * A210616 A210617 A210618


KEYWORD

sign,easy


AUTHOR

Jonathan Vos Post, Mar 23 2012


STATUS

approved



